@Serverclosed
You think that having a crazy murderous stalker is not enough to traumatize him for life? (and even longer in this case)
Said stalker killed his parents, tortured and killed any (very few) friend he made, apparently raped him repeatedly...
You don't think he would become pretty paranoid about this, regardless of the individual?
When you add that they both died and a goddess just told him: "btw, you will both be reincarnated in the same place", that would definitely be the last straw on his sanity's back.
The fact that we're talking about his sister doesn't make him a siscon, but a victim. A very traumatized victim.
I admit the transition from love to hate can be short, but those are still very different feelings. In this case, there is a world of difference, unless we start featuring Stockholm's syndrome. Which he doesn't seem to experience.