[nikdaheratik]

From a scientific standpoint, there's been alot of theory about an Aunt/Uncle advantage which is similar to a Grandparent advantage as far as it being able to help improve survival through the family of any genes responsible even if it is at the cost of an individual.

Of course, there's also the strong possibility that it isn't purely genetic but a mix of certain genes, which may be linked to other important traits, with a certain environment (possibly even in utero).

[toasterlovin]

The problem with the aunt/uncle theory is the math. A non-breeding sibling would have to enable their siblings to have two additional children (above and beyond the children they would have had anyway) for each child that the non-breeding sibling forgoes having themselves for the strategy to even hit break even, from a genetic point of view.

That's a steep hill to climb.

[kilotaras]

Because the number of siblings is close to number of children it actually comes to each breeding sibling to have extra two children survive into adulthood. This becomes even less steep if we remember that cousins can also be included in the math.

For example the gay person becomes a village healer and can spent time on that precisely because he/she doesn't have own children to look after.