I'm scared of what the sister gonna do now...
Coming to this late but without the benefit of reading ahead? She's had two tracks she wants to help her brother grow along ever since she flounced back into his life.
The first, obviously, it's the seggs. She's wanted it forever but sees he's in such a terrible state that making it happen would lead him to very, very, very bad places emotionally. Right now, even if she were nothing but encouraging in the moment and ecstatic in the aftermath, there's a good chance he'd take a knife with him into the bath after and never come back out. So that's not workable for her in the long run.
The second is that she wants him to become an actual adult who can function in society, someone she can be legitimately proud of. She's had little bits of success here and there already, but it's all been due to subtle things she's been doing for him, not things he's developed the motivation for on his own. Pairing him off with her circlemate may not have been part of her plan (though I wouldn't put it past her), but it would give him some internal motivation to properly grow up. In ways she can still mold and shape him to her liking, of course, and be there waiting when the relationship inevitably fails. She's playing a long game, after all.