Understanding shared DNA of 1,576 cM with great nephew

Question

I had my DNA tested through Ancestry. About a year ago, my great nephew (I'll call him GN-1) whom I raised had his tested. The match came up Close Family–1st Cousin Shared DNA: 1,576 cM across 69 segments. His mother is my sister's child, my niece. My sister always claimed she didn't know who the father was.

We didn't think much about it till another great nephew (2nd sister's child) tested. He tested 1st–2nd Cousin Shared DNA: 706 cM across 30 segments.

Why is my great nephew (GN-1) testing so much closer to me?

He should fall in the range of the GN-2- under 850.

My sister told me once that our own brother raped her, but denied he was the father of her child (my niece, GN-1's mother).

Is it possible that my brother fathered my sister's child and that he is actually the grandfather of GN-1??

Could that be why our DNA is match is so high?

Could my niece be my sister?

Answer 1

GN-1 could do a Y-DNA test, currently offered only at FamilyTreeDNA. The results of test that would pinpoint his fatherline, although it will not prove that your brother is that person - it would certainly narrow the possiblilities even more.

GN-1 should be sure, before sending away for the kit, that he really wants to know the answer. Such information brought into the family can be very painful and divisive.

I have chosen to know the truth about my grandfather's crimes. Few others in the family want to know.

Answer 2

In answer to your first question:

Why is my great nephew (GN-1) testing so much closer to me?

The Shared cM Project 3.0 tool v4 suggests for 1,576 cM that the relationship has these probabilities (based on stats from The DNA Geek):

• 100.00% Grandparent Aunt / Uncle Half Sibling Niece / Nephew Grandchild
• ~0% Great-Aunt / Uncle Great-Niece / Nephew

and that the:

[second] set of relationships is just within the threshold for 1576cM, but has a zero probability in thednageek's table of probabilities

While a Great-Aunt to Great-Nephew relationship cannot be totally ruled out, and you would know if Grandparent to Grandchild were the case, it seems like Aunt to Nephew is the most likely relationship.