I always wonder if it's fair to judge a character by how they acted in another version of the story because:
1. they were written that way, ( haha) but honestly
2. It's a different set of circumstances in the new story, so the character acts differently, essentially becoming a different character.
Because it's not like a linear real life that a person has lived and you want to judge them by their past behaviors and know what they have been capable of.
It is a character that is having a different experience and it is shaping them in a new way, this character never did the things that happened in the other version of the book. And he never 'loved' the sister. He was insane.
Except in the actual story he does in fact cause her older sister's death So like, he's definitely capable.
I really don't see how she could fall in love with a dude who would fall for his sister. Really. At that point the only reason he's falling in love is because they're the only important women in his life and bc they're enchantresses.