I just headed back to the early, cute chapters, and found the answer.
Chapter 1: "What's important isn't what a family should be like, but what the family wants to be like"
If they want to be a family where siblings can play hide the sausage, then so be it, that's the family they have.
(Also in chapter 1: bio-mom's "he doesn't want kids" was not a complete sentence. She was shouted down by Ryouta's father while she was saying "but Ryouta is my-", which means she was PROBABLY being less of a bitch than some of us, self included, remembered. If Ryouta's dad had let her finish the sentence, the kid might be less traumatized today. But yeah, divorces lead to angry shouting, and important things get lost in the process.)