what if children get enough rights that they can choose whether they have siblings?

This might or might not have much of an effect. I requested a new baby sister when I was four (clarifying that if it was a boy he should be "sent back"). I got one. (I honestly don't know if my request had a thing to do with it.) I proceeded to regret it for about the next decade (no longer), but she was already there. That having been said, I do plan to consult my future kids before giving them siblings or (should the situation come up) stepfamily. It'd be interesting to see how, if at all, that affects the makeup of families should the practice become widespread.