Kai is still a mystery to me so I don't really know about his background, and same goes for Sehun's parents and Jimin. Especially how Jimin once seemed like a good brother to Sehun and then later was just "his parents' son" like basically ignoring Sehun was strange, but I also don't know about his background yet. I think it's great that jinyoung cares for Sehun, it would be really strange if his brothers both ignored everything that's going on because that would be unrealistic in my opinion. I don't really know why Sehun didn't run away from home or got help though, after everything he goes through at home, there are people at school, and making his self-confidence completely disappear so he won't even bother to think of something like help is a tough thing to do so he must have been hurt by his parents for a really long time then. Why his parents act like this is another story, I don't know yet, but you should have a pretty convincing reason behind it, otherwise it would seem very strange, but I don't know about it. But I can't really judge yet because many things will be revealed in the future, and until now there are many mysteries - just make sure to make every character's acting have a good and convincing reason behind it.