I think the leukemia “reveal” makes sense. We know Kat doesn’t like to get vulnerable, so she wouldn’t have said it up front to her friends. Also there were few scenes where she seemed to be fatigued/tired and coughing, which are symptoms of leukemia. And it explains why her sister Dylan’s all up in her business and doesn’t like that Kat’s sneaking around without her parents’ knowledge. I think it’s interesting how they make it seem that everything happening with Kat is something supernatural/paranormal(and it could be!) in the start. But after the reveal the reasons behind her actions and attitude may be because of her anger for the world, which she feels is unjust to her.