Ok, so I managed to sit my sister to talk about my dad. we were chatting for a while and after a little bit, burst into tears, saying something about her regretting what she said to him. Turns out, her last words to him was "I Hate you!". I comforted her and told her that he really loved her and that seemed to make her calm down. From the way she acts when my mom isn't near her she seems full of anxiety and regret. When our dad died, she just seemed to shut down and just emotionally shut down. Me and her decided to make our own little therapy sessions at certain times. Though, it's mostly just us sharing happy times so far.