I went to high school with a girl who was I think #6 of 12. And yeah she was raised by siblings and raised siblings herself. It was actually somewhat similar to what my girlfriend experienced having a deadbeat mom and imprisoned dad.

frankly it's difficult to be good parents with one child, humans are not meant to raise even a single kid with only 2 parents. We're meant to have communities that share the load of raising all the children, we're not birds for christ's sake.

At some point, the older kids are providing child care for the younger ones. But also, its very "infinite economic growth thanks to our ever-expanding frontier" understanding of the family dynamic. As soon as all your neighbors start to copy you, and families start bumping up against one another in their effort to cannibalize the surrounding wilderness, you get conflict. Expand far enough, and the conflicts become things of history - even of legend. The War of the Roses. The French and Indian Wars. The Romance of the Three Kingdoms. The Gaza Genocide.

This is a family style game of The Tragedy of the Commons.