My mother was raped by her father sometime in her adolescence. There was a trial and he was acquitted, but it is generally known and accepted in our family that the rape did occur. I don’t know if the assaults continued after the trial.

When she was 19, our mother had my older sister. She told me that when she was pregnant in 1960, her father, who was a prominent doctor, found a man to marry her and raise the child as his own. That is the man my sister grew up believing was her father.

When my sister was in her 30’s, for some inexplicable reason, our mother told her this man is not her father, and she named a different man who turned out to be deceased.

When my sister was in her 50’s, our mother said she had lied about the second man being her father and it was actually a 3rd man with whom our mother was presently in a relationship. She said she had also dated this man in her youth. He agreed that he was, in fact, my sister’s father.

Our mother said she became pregnant with my sister when this man raped her in the back of a car. It struck us as very odd that she would then date her rapist decades later, and I have good reason to believe she didn’t even know him until decades later.

When my sister asked the man to get a DNA test to determine if he was her father, he refused. He ultimately passed away, and my sister asked our mother for strands of his hair to be DNA tested, and our mother refused. She finally admitted he was probably not my sister’s father and that my sister was just heaven sent.

My sister ended up doing an AncestryDNA test several years ago to see if she could find anyone on her father’s side. So far there are no matches other than on our mother’s side.

We cannot fathom why our mother has put my sister through this over and over. She has always been our mother’s favorite and they are very close. We also have a younger half brother, and he and I know for certain the men who are our fathers and that our mother was married 6 times. So, it’s not some misguided attempt by our mother to salvage her reputation.

It occurred to me that perhaps my mother’s story keeps changing because my grandfather may be my sister’s father. After all, he did rape my mother at least once.

I have AncestryDNA results on myself, and they show my sister as my Close Relative with 1924 cM. I understand that that’s consistent with a half sibling.

Could it also be consistent with her being my half sister and my aunt, or would it be higher in the case of incest?

My grandfather is dead, so we can’t obtain DNA from him. Our mother is still living.

If we performed an AncestryDNA test on her, would this be useful in determining if she is my sister’s mother and sister?

If so, what might we look for?

Perhaps she was actually raped in the back of a car by a man she didn’t know, so she can’t name him. I would still like to rule out our grandfather though.

1 Answer 1


Yes, it's at least possible to make this determination, although AncestryDNA by itself is probably not sufficient. Your sister inherits 50% of her DNA from her mother, and on average half of that (25%) would come from her grandfather. If her grandfather is also her biological father, then she would inherit the other 50% from him, making a total of about 75% from him. Some of the DNA inherited through her mother would be the same as that inherited from her grandfather (again, an average) - about half of the 25% that came from her mother, or about 12.5% total.

If you upload your sister's AncestryDNA sample to GEDmatch, and use the "are my parents related?" tool, the results may tell you if the biological father is her grandfather - you'd see abt. 12.5% (give or take) fully-identical segments (same DNA on both chromosomes) in her sample.

If you can get a sample from your mother uploaded to AncestryDNA, your sister would match her higher than the normal 50% (3700 cM) for mother and daughter. I'd need to do additional calculations to decide whether the match would 62.5% (4650 cM), corresponding to half of the additional DNA your mother inherited from her father, or 68.75% (5115 cM), corresponding to half of the additional DNA your sister would have inherited from her biological father (if it's her grandfather).

Also, as @lkessler has suggested for a similar question, you may want to hire a professional genetic genealogist to interpret the results for you, rather than take my word for it.

  • Thank you so much for your guidance. I am going to work on getting an Ancestry test on my mother. I will then post the results for your review. Many thanks!
    – user14001
    Sep 8, 2021 at 22:32
  • Are you able to include a link to the similar question by @lkessler, please?
    – PolyGeo
    Sep 11, 2021 at 22:38

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