How can two different generations of one family be a half sibling to an outsider?

Question

My full paternal uncle and I enough cM with an outsider to both be listed as half siblings to an unknown male. My uncle shares more cM with that unknown male than I do (a female).

Both my uncle and I are listed as half siblings to this person. I don't understand how two different generations of known lineage can be half siblings with with the same person.

Here are the numbers:

Uncle and "the male outsider" 26% shared DNA | 1,809 cM across 41 segments Unweighted shared DNA: 1,809 cM Longest segment: 125 cM 100% 1/2 sibling

Me and the "male outsider" Shared DNA: 1,655 cM across 28 segments Unweighted shared DNA: 1,655 cM Longest segment: 165 cM 100% 1/2 sibling.

My Uncle and I Shared DNA: 1,599 cM across 41 segments Unweighted shared DNA: 1,599 cM Longest segment: 125 cM 99% uncle

Can you start to unravel this?

Answer 1

Half-sibling is not the only likely relationship at that cM range, it also includes aunt/uncle and niece/nephew. So, depending on the age of the outside match, it could be either your uncle's half-brother, and therefore also your uncle; or your half-brother, and therefore your uncle's nephew.

https://dnapainter.com/tools/sharedcmv4-beta

Answer 2

One way for both you and your uncle to also be half siblings is that the same man (probably of your grandparents' generation) impregnated both your maternal grandmother and -- some decades later -- your mother.

(Note that I'm not commenting on the likelihood or the squick factor, but just how it can happen.)

Answer 3

I ran this through DNA Painter's What Are The Odds tool. The only scenario that works is if the "outside male" is your half-sibling. In that case, your uncle is also his uncle, and the centimorgan results are consistent with those relationships. No incest or other weird relationships are necessary. I considered the possibility that the "outside male" is a half-sibling or full sibling to your uncle, but those didn't work. If “outside male” were your uncle’s half-brother, that would make him your half-uncle, but you share too much DNA for that relationship. Congratulations, you found a new half-brother! See the diagram I used here.