Dear Jeanie: Half Sister Surprise

Editor’s note: The “Dear Jeanie” blog series uses fictional patient questions written by an NSGC expert based on her experiences as a genetic counselor. This series in no way represents a real patient or single genetic counseling session. 


Dear Jeanie,

My mom comes from a big Italian family and she had lots of cousins around growing up, but she and my dad moved to Texas after they got married and over the years she has lost touch with many of them.  Dad died suddenly a few years ago and I’m an only child, so it is just the two of us now.  For Christmas, I got us both those genealogy tests where you can put your results online and they match you with your relatives.  I thought it would be fun for her.

We found a couple of second and third cousins and one first cousin that mom had not seen in decades, so that was fun.  Then one day I get notified of a close match.  I open it up and it says that this woman is my niece.  One problem: like I said, I am an only child.  I send her a note, and we have a laugh about it.  It’s got to be a mistake, right?

But then I take a closer look at our results, and I notice a few things.  We do share a lot of the same results.  Also, the company says she and I have some distant cousins in common, but none of those people are on my mother’s match list.  I email her back and ask her a few questions.  She is two years younger than me and – this is very weird – she lives in Atlanta now, but she grew up in a town that’s not more than a half hour away from here.

Jeanie, is this girl my half sister?  I’ve had a few more emails from her, but I haven’t written her back.  I’m not sure I know what to say.  I guess I owe her an explanation, but I also need to protect my mom.  What’s the right thing to do?

A Confused Son (and Brother?)


Dear Confused,

We share most of our DNA sequence with every single human on this planet, but there are places in the genome where individuals vary, and genetic tests designed to identify biological relatives focus in on those variable spots.  A full sibling or a parent matches you 50 percent of the time, and a half sibling, grandparent, uncle or aunt matches you 25 percent of the time.  These tests aren’t always perfect, but they are very reliable at identifying close relatives.  Logic suggests that this woman cannot be your aunt, your niece or your granddaughter.  It is safe to assume that she is your half sister. 

The results indicate that you and this woman have the same biological father.  Your letter suggests that you assume this must be your father’s child from an affair.  Keep in mind that given what you present here, it could be the reverse – your mother could have had a relationship with her father.  It’s also possible that there was no affair and that your parents or her parents used donor sperm.  You are probably in a better position than me to judge which of these scenarios is more likely. If you choose to share your suspicions with this woman, the two of you may wish to consult with a specialist in genetic genealogy, who could help you confirm and clarify the origin of your match.

Okay, big breath.  You have a dilemma in front of you.  You ask “what is the right thing to do?” and I want to say up front that there is no simple answer to that question.  There’s no one right answer.  That’s frustrating, but it’s also liberating, because you are free to do the thing that feels right to you.  There are things to consider, so let’s look at some of the potential issues, assuming, as you do in the letter, that this is your father’s child.

We don’t live in a tribal society, and you have no special DNA-based obligation to this young woman, who is your blood relative, but also a stranger.  That said, most of us would help a stranger if we could, if that person was in danger.  

We don’t live in a tribal society, and you have no special DNA-based obligation to this young woman, who is your blood relative, but also a stranger.  That said, most of us would help a stranger if we could, if that person was in danger.  So one thing you should consider is whether or not there is any family health history information on your father’s side that she needs to have.  I’m drawing a line here between routine information that might be useful but isn’t life-altering, and something like a family history of early onset breast cancer or sudden cardiac events.  If you can give her information that might save her life, then that would be a compelling reason to respond.

Now your mother, who is not only your biological relative but the woman who raised you, certainly deserves a different level of respect and consideration.  It’s possible that this will be less of a surprise to your mother than it was to you and it’s possible that she may be less upset than you anticipate.  Most likely, however, it would be a hard conversation for you both.  You can choose not to go there.  One thing to keep in mind is that you may not be the last member of your family to experiment with genealogy testing.  You can bury this now, but you may not be able to bury it forever.  So you should think about whether it would be better or worse if it came up at a later date.

There’s a third person here who’s feelings should also be respected, and that is you.  You don’t tell me anything about your own thoughts and desires.  You’ve never had a sibling.  You lost your father several years ago.  I can only guess, but it would be very natural if you were curious about this woman, who shares that close biological connection to you and your dad.  I don’t think you have a responsibility to follow up with her.  But I would understand if you did. 

My final piece of advice is this: take your time.  You don’t need to rush.  Talk to a close friend or family member if you feel that will help you.  You can reach out to this woman now, or in a year, or never.  The biology is not going to change, but how you feel about it may evolve.

Yours very truly,


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