I've seen this hypothesized and studied before as how homosexuality exists in the face of evolution. That is, some combination of (a) without children of their own homosexuals are better able to help raise their other family members, (b) homosexuality is strongly correlated to higher fecundity of female family members, and (c) homosexuality is just a fluke in sexuality in general, similar to how other complex biological processes often have birth reflects related to some failure in each step of those processes.
While the "fertile siblings" theory and evidence make a lot of sense to me, I also find it somewhat depressing, as if being gay is just kind of a fluke byproduct to help your other family members.
Yeah, evolution is a bitch. Never ascribe moral worth based on what evolution favours.
I think the theory makes more sense in communities in conflict. Missing fathers make the presence of an uncle much more important and high mortality rates make fertility rates more important.
Interestingly that somewhat overlaps (to my untrained eye) with cultures more accepting of homosexuality: early Japan, ancient Greece, modern tribal societies.
While the "fertile siblings" theory and evidence make a lot of sense to me, I also find it somewhat depressing, as if being gay is just kind of a fluke byproduct to help your other family members.