Parents can’t pull fast ones on the kids, preserves family history more coherently, lowers risk of incestuous parent-child emotions developing.
Part of the horror of modern narcissism is the way in which parents cheat their child or “perfect two” children out of access to a continuous family history and further leave them vulnerable to emotional manipulations that are much harder to do when all the kids can compare notes. You can run into problems with ten or fifteen siblings, but human history isn’t average people having that many kids as the normal family size.
I hope to explore this more in written form, one of the downsides of notebooking it in blog format. Siblings are important because when the parents are outnumbered, they kind of need to communicate to the children that there are other people who care about them and can take care of them too. The benefits accrue just having three kids instead of two, but are probably sweetest of spot at 4-6 kids. After that, the sheer numbers issue becomes a massive problem if the kids aren’t raised factory-style with lots of other adults around. It’s easy to get paranoid and make the child or children believe there’s nothing outside the nuclear family unit when there’s only one or two kids. It’s harder to be anti-social and atomistic in relation to one’s local community when you have to do a name-check to make sure all the kids are rounded up.
Anyway siblings are valuable enough that society should be ordered for women to be able to have three or more children without it being a heavily politicized, rare choice.