Our Guide to DNA Tests for Siblings

Updated December 20, 2018

This article was scientifically reviewed by YourDNA

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A list of references is also included at the bottom of this article.

Is there a DNA test for siblings?

Yes. A sibling test analyzes the DNA of two or more people to determine if they share one parent, two parents or no parents.

These produce half-sibling, full-sibling or unrelated results.

What's in this Guide?

Disclaimer: Before You Read

It is important to know that your genes are not your destiny. There are various environmental and genetic factors working together to shape you. No matter your genetic makeup, maintain ideal blood pressure and glucose levels, avoid harmful alcohol intake, exercise regularly, get regular sleep. And for goodness sake, don't smoke.

Genetics is a quickly changing topic.

What is sibling DNA testing?

In many cases, a sibling DNA test is performed when a standard paternity test is not possible.

Sibling DNA tests are performed for many reasons, ranging from reassurance about relationships with a person’s siblings to tests that need to be used as part of documentation that can be admitted in court.

“Peace of mind” tests are available online, but for a “legal” sibling test, it will be necessary to have the test carried out by a government approved lab testing service 1.

How do sibling DNA tests work?

The first step in a sibling DNA test is determining why someone wants to have a test done.

It may be a known fact that the siblings are sure they do not share the same mother, and they want to find out if they share the same biological father, or they share the same mother but are not sure if they have the same father.

It is important to make the testing lab aware of this information so that the correct sibling test can be performed.

To get the most conclusive results, if a person knows for sure that they definitely share one parent with a possible sibling, that parent should be included in the testing process.

This helps to distinguish between DNA from the known parent and DNA that you might share with the alleged parent.

Genetic markers from the tested parent helps to reconstruct parts of the DNA from that parent as well as the sibling in question. When one parent’s DNA is included, then it is possible to see what part came from the other parent.

With a peace of mind test, each person participating in the test will receive a kit that will contain swabs that allow for the collection of “buccal” samples from the inside of a cheek.

Collection is simple, painless and only requires lightly brushing the inside of a check for about 10 seconds to collect a sample.

After a cheek has been swabbed, the sample is placed in a container and returned to the lab you are using.

The goal of a sibling DNA test is to achieve at least 99% accuracy to prove or disprove the relationship between the two people taking the test.

Types of sibling DNA tests

In most cases, sibling DNA tests will compare two scenarios to try and determine if individuals share one or both parents in common.

Two people who share the same biological parents are called full siblings. If one parent is shared, then the two are known as half siblings. If neither parent is shared, then the two people are unrelated.

Full Siblings vs. Unrelated

In this type of test, the DNA of two people is tested to determine if they are full siblings who share the same mother and father or if they are completely unrelated.

It is most commonly used in immigration cases where a U.S. citizen wants to sponsor someone they claim to be a sibling who is applying for an immigration visa.

Full Siblings vs Half Siblings

This type of DNA test is performed when it is known that two individuals have the same biological mother and there is a desire to know if they also share the same biological father.

For this test, it is strongly recommended that the DNA of the mother is tested as well as the two individuals.

This will allow a lab to determine which genes were inherited from the mother, eliminate them and then compare the remaining data to see if there is a strong DNA match coming from the same father.

Half Siblings vs. Unrelated

This DNA test is performed when two people know they do not share the same mother but want to find out if they share the same biological father.

Testing for the same father is done in the vast majority of these cases, but it is also possible to test for the same biological mother as well.

It is helpful to have the DNA of the common parent also submitted so that the parent’s portion of the DNA can be eliminated from each sibling’s genetic profile.

What percentage of DNA do siblings share?

While it’s true that children inherit half their DNA from each parent, the percentage of DNA passed from one parent to individual siblings can vary widely. DNA is not passed down from generation to generation in a single large block.

This means theoretically that two siblings could have quite different ancestry results from a DNA test even if they do share the same parents. It has to do with slight variations in egg and sperm DNA.

Every time the body creates an egg or sperm, genetic recombination takes place. Basically, before fertilization, each cell has 46 chromosomes, but when the sperm and egg combine, the number of chromosomes is cut in half so that when the egg and the sperm combine, they form a single and complete genetic package.

Each time this happens, there is a unique recombination that takes place, producing individualized DNA for each offspring.

Because of recombination, at best siblings only share about 50% of the same DNA 2, meaning that their genetic codes could be much different for at least one or more of the markers that is analyzed in a sibling DNA test.

Can a DNA test prove half siblings?

In many cases, it is better to say that a DNA test can provide a strong statistical probability of whether or not two people are half siblings or unrelated.

If two people know for sure they have one parent who is different, but are unsure if the other parent is different, a half sibling test can be conducted.

The results will give a strong indication one way or the other, but because DNA passed to each child from a parent varies by child, testing companies will likely hedge a results by using the terms “highly likely” or highly unlikely” when reporting results back to test subjects.

Because there is a range in DNA matching among siblings, to gain more accuracy if may be possible to test additional relatives to give a more conclusive result.

Are sibling DNA tests accurate?

Sibling DNA tests are accurate, but not as accurate as paternity tests. This is because siblings are not as closely related to each other as they are to their parents.

Tests compare DNA markers from both people taking the test and analysis takes place at certain DNA locations to try and establish a genetic match.

Most test providers check 16 locations, but it is possible to check up to 68 locations and that will improve the accuracy of the test.

Requesting a check of more locations will cost more but may be worth it depending on your reasons for the test.

When two siblings know for sure they share one parent and want to confirm if they share a second parent, accuracy will also increase if the known parent of both siblings is tested.

This helps to identify and exclude 50% of the child’s DNA that came from the known parent so that the focus can be placed on the DNA with the other possible parent.

Passing specific DNA from a parent to a child can vary widely from child to child. Because of this variance, the amount of DNA that siblings share can be much lower than the amount of DNA shared between a parent and a child.

That means there are fewer markers to compare which results in the accuracy of the sibling DNA test being less than 100% certainty.

As a result, when results are returned to test subjects, they often times come with an assessment of how conclusive the results are. Many times, this is expressed as a Combined Relationship Index 3 (CRI) that states a numeric value of 0.00 to 100.00.

CRI values in a paternity test are either expressed as 0.00 (meaning no biological father relationship exists) or as 100.00 (a biological father relationship has a high probability).

However, in a sibling DNA test, because there is not as much DNA that is shared in common between siblings, CRIs often are expressed somewhere in a range between those two absolute numbers, giving less definitive results.

Can sibling DNA tests be wrong?

It is possible to come up with wrong test results in a sibling DNA test. Some of this will depend on the level of complexity of the test that is performed, and some will depend on the lab itself.

DNA is generally split into 23 pairs of chromosomes and each chromosome is one long strand of DNA.

There are more than 6 billion parts of a person’s DNA that can be looked at, but a standard DNA test will only look at a small fraction of these spots (known as markers) to see how many of them two people have in common.

Based on this analysis, it is determined if two people are related or not and what the probability is of the relationship.

Because half siblings only share about 25% of their DNA in common, when only a few markers are checked, the sample size is so small that it may impact the accuracy of the test.

In some situations, it is possible for two unrelated people to share the same markers with each other as two people who are related. This is why also having a parent involved in sibling DNA testing helps with accuracy.

By comparing the parent’s DNA with each possible sibling, it is possible to rule out some of the markers that don’t match up.

One way to decrease the chance that a sibling DNA test can be wrong is by significantly increasing the number of markers that are checked to allow for a greater confirmation.

When the number of markers increases to hundreds or thousands to be checked, then there will be a greater chance for comparison and confirmation or denial of a sibling relationship.

It’s also critical for a lab to use good practices to handle samples when they perform tests. Most of the time this is not a problem, but on rare occasions when a lab tech does not follow testing protocols related to cleanliness or possible cross-contamination, mistakes will happen.

How much does a sibling DNA test cost?

In general, the costs for DNA tests will vary based on a number of factors. The good news is that overall, DNA testing costs, including sibling DNA tests, have dropped quite a bit in recent years, making them much more affordable than ever before.

Paternity tests are the most basic and so they will typically cost less than those for ancestry reasons. An in-home, non-court approved paternity test can run as little as $30.

However, when the paternity test is court ordered, it can run as much as $500, so the reason for the paternity test will have a big impact on the cost. In many cases, results are available on the same day if you are willing to pay a premium.

More often, results will take a couple of days and is what the vast majority of people choose. If price is a concern, you can always shop for a lab that will take a longer time to provide results (perhaps a week or so) and they will usually price their services lower than other facilities.

Testing for disease markers was once limited to patients who were concerned whether or not they had inherited certain diseases. This type of test has also become much more common and covers a wider variety of diseases than ever before.

However, testing for disease markers still costs more than other DNA tests due to the extensive nature of the tests and the complexity involved in interpreting the results.

Another thing to consider is that you might want to take a low-cost in-home test to get paternity answers. Results will not be admissible in a court of law because you did not provide a sample in front of a medical professional, but this test will give you a strong indication of paternity before you decide to take a case to court.

It may also help you to decide if you should take a legal test to support your case as well.

Ancestry DNA testing has become the most well known of all DNA tests thanks to the popularity of 23andme.com and other similar companies offering this type of testing.

DNAtestingchoice.com provides listings for every test you can buy online and take at home as well as pricing that ranges from free to testing that costs over $1,000.

Pricing on the high end will include DNA tests that include testing for genetic and health markers as well.

Most companies provide results in anywhere from a couple of days to 12 weeks.

Testing for health alone will run anywhere from $100 to $500 depending on how extensive the test is and what lab you decide to use.

There are many potential uses for the Sibling DNA Test

Immigration Visa 4. United States Citizenship and Immigration Services (USCIS) recommends DNA testing to prove biological relationships for people who are citizens or lawful permanent residents of the United States who need to establish a relationship with certain alien relatives who want to immigrate to the United States.

People attempting to assist an alien relative who wants to immigrate will usually file a Form I-130 as the first step in this process. Immigration DNA testing helps to expedite the application and approval process.

As an initial part of the application process, an applicant will receive paperwork from USCIS or other agency that requests DNA evidence of relationship between the petitioner and beneficiary.

Samples of the U.S. citizen must be collected at an approved AABB (American Association of Blood Banks) laboratory. DNA samples are collected at the lab and the DNA collection kit is then shipped to the U.S. Embassy/USCIS office in the applicant’s home country.

The Embassy/USCIS collects the DNA sample and ships samples back to the testing lab. After the test results are complete, the lab sends those results to the requesting immigration office.

Once the samples are at the immigration office or Embassy in the applicant’s home country, the applicant will be contacted and will supply a DNA sample collection at an appointment in their home country.

The results between the two tests will be analyzed to determine if there is a biological match.

As of April 2018, positive test results for sibling DNA tests or half-sibling tests are accepted as proof of relationship as long as the testing was conducted through an AABB-accredited lab.

Results for a full- or half-sibling DNA tests of 90% probability or higher are considered evidence of the claimed relationship.

In most cases, results between 9% and 89% probability of relationship are considered inconclusive and additional affirmation from the lab may be needed to establish the relationship.

This is done either by testing extra genetic markers or by including more relatives in testing.

Social Security Benefits. In cases where a single mother is not getting Social Security benefits for their children because the father or alleged father is deceased, it may be possible to perform a DNA paternity test.

If an autopsy was done when the father passed away, a sample of the father’s blood can be collected from the state medical examiner’s office in the state where the autopsy was performed.

The blood sample will be sent from the coroner’s office to Social Security where it will be compared with the mother and children’s DNA which will also need to be tested as well.

If an autopsy was not done, then one or both of the father’s parents can be analyzed through a Grandparentage DNA test. If the grandparents are not available, a relationship test can be performed if the father had a brother, aunt or uncle on the paternal side.

Assuming paternity is proven, when completed, you can contact a local Social Security Office to make arrangements for children to start receiving Social Security Survivor Benefits.

You will need to bring a birth certificate, death certificate and an original copy of the DNA results.

Personal Knowledge. As DNA testing has become more widespread and affordable, many people simply want to clear up any issues regarding sibling or paternity relationships for a variety of personal reasons. At-home DNA tests are commonly used for this purpose, not only to establish relationship links, but as a means to track ancestral information as well.

At-home tests provide the same information as legal DNA tests, but these non-legal tests are not intended for use in a court of law 5 because samples are not collected in a controlled and supervised environment.

At-home tests are readily available online from a number of suppliers or at pharmacies nationwide. Walgreens, CVS and Rite-Aid sell in-store tests for $25-$50.

This is for the collection kit only. There is an additional fee when the collection sample is mailed to a lab for analysis and results.

Would Sibling DNA results be recognized in court?

Sibling DNA test results can be used in court cases, but for the results to be admitted as evidence, collection must follow a strict chain of custody and testing must be done by an accredited AABB laboratory.

Typically, DNA tests can be used to support claims in child custody or child support cases or in certain immigration cases. Legal results are recognized in state, tribal and federal courts and by Immigration and Naturalization Services.

Some people may be legally required to take a DNA test to show the relationship between a child, siblings or an alleged parent.

Depending on the jurisdiction, people who can request a DNA test may include an alleged mother or father, a legally recognized mother or father, a legal guardian, government social services personnel or a prosecuting attorney.

Some states have statutes of limitations when it comes to requesting or requiring a DNA test to be performed, and tampering with the results of a DNA test can result in punishment that might include fines and jail time.

When should I use a legal sibling DNA test?

A legal sibling DNA test should be used when attempting to secure Social Security Survivor benefits and in immigration cases.

In some instances, it may also be used to establish a relationship for inheritance or probate issues.

Choosing the best sibling DNA test

You have many options when choosing a sibling DNA test and which one you decide to use will depend entirely on your personal circumstances.

You should first decide if you want a test for personal knowledge and peace of mind only, or if you will need to have a legal test performed that could be used in a court of law. The science is exactly the same for both.

The only things that are different are the chain of custody in collecting and transporting the sample, and that testing must be done by an accredited lab.

If accuracy is a big concern, then look for tests that compare the most markers. Some tests may only compare a small fraction of DNA, as little as 16 or 24 markers.

Other tests may look for long blocks of DNA that are identical in both subjects and as a result will compare as many as 700,000 markers. Keep in mind that for sibling DNA tests, no test will be 100% accurate.

Testing will indicate a high or a low probability that could range up to 99% down to 1% or at some range in between. Labs must rely on statistical formulas to calculate those probabilities and tests can be inconclusive in some cases.

When shopping for a DNA test, also be a bit wary of low priced kits. The price of the kits may only include the kit itself and not additional costs that you will have to pay for lab services and shipping.

Many labs are located in the United States, but some are located overseas, and this could not only raise the shipping costs, but it means that getting results could also take longer as well.

Based in the UK, DNA Testing Choice is a news and reviews site for at-home DNA tests. The site has a comprehensive listing of DNA tests as well as information on the latest genetic research from around the world.

The site provides testing research and products into language that is easy to understand and does a good job of providing objectivity for DNA testing providers and their products.

To compare DNA testing kits and services, visit their website here.

Referenced Sources

  1. AABB Accredited Relationship (DNA) Testing Facilities
    AABB Accreditation Department.
  2. Autosomal DNA statistics
    International Society of Genetic Genealogy.
  3. Interpretation of DNA Typing Results for Kinship Analysis
    Kristen Lewis O’Connor, Ph.D. National Institute of Standards and Technology
  4. DNA Relationship Testing Procedures
    U.S. Department of State
  5. Paternity Proceeding Establishes Parent-Child Relationship
    Ohio Bar Association - Karen Riestenberg Brinkman