MC is like 15/16, there's not much you can do in that situation. They already disregarded their kids's feelings by putting off telling them until the day after they signed the papers. And honestly I'm sure that both are still processing the fact that their sibling is NBR. Suddenly finding or losing family easily blocks your ability to logically reason about.
Also: 15/16 is a good age to rebel, he should've started then. ¯\
(ツ)/¯
But honestly, it shouldn't be an issue even if she stayed with his brother and the other parent. Like they lived together for 12-some years, they can't just suddenly ignore they've been together for this long, surely they should treat them like their own kids, whether they're biological parents or not, otherwise it's weird.
If they were to move away, they should've at least move with the new school year and not in the middle of her last year of middle school. Are they seriously thinking she's gonna be alright with her exams transferring all of a sudden in the middle of last year of middle school?
They also did mention last chapter that they may feel part of the divorce's purposes is to explicitly separate the two. We don't know how much the parents really know of their relationship, but bringing up the topic if they are aware will lead to some very awkard talks that I imagine neither want to have (even if ironically enough, the parents removed every barrier that would make such a relationship awkward).
Biggest factor there I'd bet based on the past chapter is their parents are well aware that both siblings were too close for just familial relationships. Any negotiation that would hint them being together especially without parent supervision would've been binned if that was the case. He could've been rebellious and angry about it, but I don't think that's his type of thing to do and it's not like they both resent their parents that much either to do so.
As raze said, if they get divorce to separate the two, they may just as well let them stay together anyway since they're not gonna be bound by any "it's your sister so you can't".
They can still line each other right? If they want to, they can still get together.
And if he had not resented his parents, well, now he has a reason to.