Anecdotal, but I think it's that one of the children gets the brunt of it and the others adjust. In my case, my elder brother had it the roughest, also since my parents were young and it was their first; I "compensated" for his more rebellious behaviour by not doing all the things he did "wrong". There's also something at my ex partner's case but I can't speak for her, but I can say that even though she got it worst, all her siblings ended up messed up in some way.