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I have been working on finding my biological grandfather. The closest DNA match I have to my father (who has taken a test) is 1015cM. I've narrowed down his father to either her father or her uncle. I am certain that it is one of these sons as we're related to both her grandfather and one of her half great uncles from her grandmother's second marriage, therefore related to one of the sons from her grandmother's first marriage.

Am I going to the right direction?

If she's potentially my father's 1/2 sister or 1/2 1st cousin, then the 1015cM seems wrong.

Yes, the closest DNA match is a woman who I am referring to as she.

Let me see if I can explain this better. My DNA matches lead to a single paternal last name-over 37 matches and counting. I have now isolated this to a married pair, matching both to the male's decedents and the decedents of the female's second marriage, thus their 6 children. The women are ruled out as I'm looking for my biological grandfather. This leaves 4 boys. Two never were in the US at the right time, the other two were. The 1015cM match is to the daughter of one of these boys (Thomas), plus I match to his granddaughter and great granddaughters from another marriage. I'm trying to narrow down if he is actually my grandfather, or if it's his brother (John) (Whose decedents I'm trying to track down.) My question is, does that 1015cM line up?

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    This is a little bit confusing. You've introduced a female pronoun without saying who "her" refers to. One possibility occurs to me. Does "The closest DNA match I have to my father (who has taken a test) is 1015cM" mean "The closest DNA match I have found to my father (who has taken a test) is a woman who shares 1015cM with him"? That would make the rest of this question make more sense.
    – shoover
    Sep 20, 2020 at 5:23
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    I think a picture illustrating your two hypotheses would also be helpful.
    – PolyGeo ♦
    Sep 20, 2020 at 7:38

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I took a look at this on the DNA Painter Shared cM Tool. I see why you ask: 1015 cM is too much to be a half 1st cousin, but too little to be a half sibling. Normally, if the choice is between two brothers, having a test of one of their children (like you do) would definitively tell you which one is the father. But, given the results, you will have to re-think your analysis.

First: Is it possible that John and Thomas' father is also your grandfather? That would make Ms. 1015cM your dad's half-Niece, which fits.

Second: If there is unexpected endogamy (marriages between cousins, say) in that family, that could make the relationships appear closer than they are. So, perhaps, Ms. 1015cM is your dad's half 1st cousin AND 2nd cousin. That could explain it. Was Thomas' wife (Ms. 1015cM's mom) also related to the family somehow?

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