Father and Nephew

[Glen in Tinsel Kni]

The more results you can gather the better and knowing how much dna they share with each other as well as with you is useful information too. It all helps to create an accurate tree and makes life easier when trying to figure out mystery matches or solve an NPE/missing ancestor scenario.

[GfL]

Thank you all, fingers crossed I might fathom something out.

[Glen in Tinsel Kni]

There's a coverage estimator that can be useful, it not only estimates the dna coverage of an ancestor but can suggest the most beneficial target tester.

Link to the coverage tool and an example showing coverage/suggested tester;

https://dnapainter.com/tools/coverage/examples/e1

[GfL]

Thank you Glen, I best get asking my cousin to do a test.

[Ayashi]

You didn't mention what made him suspect the sister was the mother?

If I'm following this right, the suspicion is that his father had an affair with his sister-in-law and his wife adopted the child as her own?

Assuming the two sisters had the same two biological parents they would have the same DNA ancestry but if that's not the case that would be something to look at.

Despite his feeling that his father was biological, I would also look further afield with the father just in case. If you can't DNA match to his believed father's ancestry then you know something is up. A match to the sisters but not the father might suggest that he was the illegitimate child of the aunt by an unknown man.

[Wexflyer]

Assuming the two sisters had the same two biological parents they would have the same DNA ancestry but if that's not the case that would be something to look at.

This is what has been on my mind. Assuming that either one of OPs scenarios is correct, then irrespective of which sister was the mother, the child will have the same four grandparents. So is DNA testing going to resolve this question? I am not sure.

[Ayashi]

Assuming the two sisters had the same two biological parents they would have the same DNA ancestry but if that's not the case that would be something to look at.

This is what has been on my mind. Assuming that either one of OPs scenarios is correct, then irrespective of which sister was the mother, the child will have the same four grandparents. So is DNA testing going to resolve this question? I am not sure.

Either dad's sister is a full blooded sister or is a very closely related half sister (same father, closely related mothers) or, "best case scenario" (from a certain perspective) a normal cousin. In either of the first two possibilities, I'm not sure how one would be able to differentiate conclusively from DNA testing a child of hers. I don't know how you'd know without also having both mothers to compare against.