Some feary has given the power to Johan? Who and why?
Probably Johans sister will seek comfort with Johan, I really want to see them being friends and enemies to the world... How? Why?
Have you ever seen the movie Disco Pigs? It's a Scottish indie movie, if I recall.
It's about two almost-sibling, almost-lover teens who lose themselves in their own reclusive fantasies. It's about what happens when she tries to pull away, and he gets violent.
Disco Pigs would give you amazingly spot-on ideas for how to handle Johan & Sister, as well as just being generally good fodder for Monsterhearts.
...I'll reply to the rest of this stuff soon. I just wanted to toss that movie at you before I forgot to.