> Might as well be raising a dog and a dolphin.

That made me chuckle.

But it's possible for siblings not to share any genetic material right?

A brother and a sister would obviously get different genes from their father and presumably there would be a 50/50 chance of them getting different genes from their mother.

There's four unique prototypes of children any two people can produce together.

My experience as well. My two are so different it’s hard to even parent at times as they need different kinds of attention. You can’t really give yourself in different ways at the same time. We recognize that one - being born in the lockdown environment - is likely different in meaningful ways than our first. Our first is helpful in she almost recognizes a difference.

Fraternal twins as well -- night and day.