This one is really interesting. Primarily speaking ants a good example of where this type of genetic expression is taken to its logical extreme.

It's basically the "aunt" and "uncle" survival trait that shows up in communal creatures. Homosexuality shows up in creatures that both need to raise their young and stay together in herds. So humans of course very much satisfy this condition.

So how's this relate to ants (and bees and termites too)?

In a colony, technically speaking everyone is the queens offspring, so everyone are siblings.

The queens aletes (prince and princess ants if you will) are only the queens offspring, all the drones dont sexually reproduce.

Yet, the drones all care for the offspring as if they are their own, that's kinda weird from a survival standpoint right?

Well it's simple, the drones are clones of the queen, so they also heavily share genetics with the offspring. From a genetic standpoint there's no difference.

So if the drones take care of the offspring like their own, and the offspring go on to mate, then the drones genetics also propagate. They have a evolutionary pressure to raise the babies despite not having any of their own.

The same occurs for homosexuality. Human babies are a lot of work to take care of, and if you have a sibling you share a lot of genetics, then you have an evolutionary pressure to take care of your nephews and neices. It's the "next best" option to propagating your own children. The genetic difference isn't much worse.

So what ends up happening is you have an evolutionary benefit if a percent of your population is born homosexual, as they will help with raising their nieces and nephews, and it turns out this combo has a net higher survival rate than just everyone being hetero.

So over thousands of years we get this gradual pressure to settle on a sweet spot of some % of us being gay.

This is also a solid explanation for the "fifth brother" thing, where every son a mother has is exponentially more likely to be gay. We haven't fully isolated what causes it (its prolly hormones) but it's a well known occurring phenomina that the more older brothers you have, the more likely you are to be gay.

Based on the above supposition, it'd 100% make sense from an evolution standpoint for mothers to produce homosexual offspring if they have multiple healthy children, as after a certain point it makes sense strategy wise to have a couple gay uncles/aunts that naturally help their older siblings with childcare.

It's just way way better for survival in nature if not everyone is pumping out babies, and a handful just aren't into that but still take care of their family.

Basically it's quality over quantity!