What is a Sibling DNA Match?

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Have you found a close match on your DNA test results?  Maybe you are curious to know whether it could be a full or half sibling who is unknown to you?  Find out how much DNA you share with a sibling.

On Ancestry DNA, and most other testing sites, they won’t be able to tell you that your DNA match is a sibling.  The only relationship that they will be able to tell you with 100% certainty is a parent/child relationship, due to the extremely high percentage of shared DNA. 

The rest of the relationship calculations are based on shared DNA percentage, or centimorgan (cMs) amounts.

How to Know if I Have a Sibling on My DNA Match List

Step One:  Find out how to see your amount of shared centimorgans with your match here.

Once you know how many cMs you share:

Step Two:  Each family relationship falls into a different category of shared DNA.  For every relationship, there is more than one type of relationship that can be included in that range.  Compare your results with the averages below:

• Identical twin sibling AND parent/child:  3400 cMs (average), ranges from 3300-3700 cMs
• Full siblings:  2550 cMs (average), ranges from 2200-3300
• Half-siblings: 1700 cMs (average), ranges from 1300-2300 cMs

If your shared DNA range falls within the full sibling category, congratulations!  You have found a full sibling. 

There is no disputing this fact, though some still will try.  It’s possible to have a full sibling that you don’t know about – there are many situations that could cause an event like this to occur. 

The most important thing to do now is determining how this happened, and to make contact, if you would like to.

Step Three:  Take a look at your match’s family tree, and see if you can find information about them online.  Do a quick search for their username on Google, or see if you can find them on Facebook.  

For matches that are other than a full sibling match, you will need to use other information in order to determine the exact relationship.

How Do I Know if I Have Found a Half Sibling on My DNA Match?

If you share between 1300-2300 cMs with a match, I can tell you one thing for 100% sure:  You have found a close family member.   With 99% confidence, you have found one of the following:

• a half-sibling
• an aunt or uncle
• a “full” niece or nephew (i.e. the child of one of your full siblings)
• a grandparent or
• a grandchild

So if you do have this mysteriously close match, and the shared DNA falls within the half-sibling DNA range, you have to also consider that this person might be related to you in one of the above ways.  No matter what, it’s exciting – but there is still work to do to figure out the exact relationship.

How to determine if your match is a half-sibling or another relationship

1.  Can you determine where this match was born, or where they are from?  For example, if you know that your father sometimes traveled for work and was often in Chicago, for example, and you have high cMs DNA match in Chicago, that can be a clue that it is a sibling.
2. Can you figure out about how old your match is?  If the match is much, much older than you think that a sibling, or even a parent should be to you, it could be a grandparent.  This is especially important if you don’t know who one of your biological parents is – it is entirely possible that one of your grandparents has done a DNA test.  A grandparent will share the same amount of DNA with you as a half-sibling.
3. Look at the matches that you share in common with this unidentified match.  Do they match on your mom or dad’s side of the family? Where do those matches live?  This could give you a clue as to who their parents are, and thus, how you are related.
4. If you have a full sibling, consider the idea that they might have had a child that you aren’t aware of.  A child of a half-sibling, however, will share 50% less DNA than a full niece or nephew.
5. Consider contacting them, just saying something about how “you’d like to figure out how you are related”.  I’ve written a post about how to contact DNA matches, if you think it might help.

The more DNA matches you have, the more you can learn about your family

If you have a DNA match show up that you weren’t expecting and none of the methods above seem to work to help you determine exactly how they are related to you, you might consider the following actions:

• Uploading your DNA to other sites to get more matches
• Doing additional DNA tests to find new matches

Conclusion

Have you had a close match show up, and you don’t know how they are related to you?  Have you found a sibling through your DNA test?  I’d love to hear from you in the comments.

Thanks for stopping by!

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