>nepotism is a natural mammalian instinct, it propels us to take care of our children

Plenty of primates and human groups have shared child rearing in a non-familial way. Tribes were not aligned exclusively on family lines, and "it takes a village" was a literal statement.

Humans have an instinct to take care of babies, not just our own progeny. Our pets literally evolved to take advantage of that. A cat is not at all your genetic family member, and yet will still trigger child rearing instincts in tons of people.

This idea that we are only programmed to take care of direct genetic relatives is incorrect and a societal choice, not a scientific one.