Mr. "Hotel chain owner" recently became Shio's step-dad. Which I guess means that Shio became a rich kid without realizing it.
I wonder why they moved into Shio's mom's house, and not Kafka's dad's house, or just got a new place altogether if he's that loaded. I guess the easy answer is that the parents don't really care since they're not living there anyways, lol.
Also, this made me realize that I don't know which family the Amamiya surname comes from. As far as I'm aware, Japanese surnames are patronymic, which would suggest that it came from Kafka's side. But the way Shio said her name first in Chapter 3 kind of just made me assume that it was Shio's original surname.
Speed reading through the chapters again though, it looks like Kafka has 'Amamiya' written on her test paper in the flashback of chapter 27, so I guess that solves it, and it is Kafka's surname as one would expect.