1760 cM half sibling or double cousin

Question

My half first cousin and I have a bit of a mystery. We are afraid of the answer, honestly.

Some history: his mother and my father are half siblings who share a mother. We do not know who my paternal grandfather is but we do know both of my aunt's parents. She was given up for adoption but raised with fairly decent contact with her biological family. My aunt has one son who should be my half first cousin, due to having different grandfather's. He, however does not know who his own father is. Therein lies the scary part. We have 1760 cM match which tells me he is either my half brother or possibly my uncle. The half brother doesn't seem right (beyond the inherent yuck factor) because it seems like we would have a potentially even higher match because we would compound cousin with half sibling. Like a double cousin raises the cM match. Plus, the timeline doesn't work. She was living in Oklahoma at the time he was conceived and my dad was in CT raising his 6 year old daughters. He never went out there and she didn't come visit.

To throw another wrench in the works, my father has other half siblings scattered around the state and country from his mother. One of which was close with my aunt, his half sibling as well. Could this be the result of a double cousin looking more like a half sibling? Or perhaps, he really is an uncle with my father not knowing his own father.

The facts are:

• We share 1760cM over 62 segments
• Autosomal DNA comparison shows 58 Pct SNPs are full identical
• X-DNA shows 1 segmy at 19.7cM
• Gedmatch believes my cousin parents are related at 405.7cM

His mother (my aunt) is deceased and I haven't worked out how to bring it up to my dad. I'm just curious if this definitely looks like it could be a half brother or possibly something else. Thanks.

Answer 1

OK, so am I understanding it correctly that you have the following set of relationships:

  ?1 --- A --- B
      |     |
C --- D     E --- ?2
   |           |
   F           G

where F is you, and G is your cousin?

The easiest explanation for a match of 1760 cM between F and G is if ?1 = B, that is, your dad (B) is actually your aunt's (E's) full sibling, not half. This doesn't address the identity of ?2 (your cousin's dad); he could be anyone.

Absent a version of DNA Painter that takes double cousins and other variations into consideration, the best way I can think of to test the hypothesis is to test D (your dad) and compare him to G (your cousin).