Honestly I don't think it's about sex attraction towards her, it's more of him being traumatized by what he saw, without being properly discussed with therapist. So he might think that everything sexual is rape, and so he thinks that his sister is still being harmed
I know he said that because he is under hypnosis, but the alternative explanation could be "we aren't related our parents just remarried so are you a homophobe?"
This is such a good story