'The only trustworthy caregiver wasn't there for me when I needed them the most' was prolly one of the most impactful emotion he felt during the whole fiasco as a child and since he knows his sister wouldn't abandon him he subconsciously felt 'safe' enough to blame her for his trauma and lash out. He 'hates' her but he doesnt hold the same reaction to her as he would his cousin(the real abuser). Its really immature but makes sense for a teenager.
I guess you are right but telling her to kill herself is too much?
Lol yea
I get that he is coping by blaming her but why her? She wanted to help you but you never told her what happened, I get why his aunt told him not to but still how is it her fault ...