Daddy turned on when watching daughter masturbate

Ever since her mother passed away she's been acting up. I know I'm a little over-protective for a step-father, but she's my little girl. She's turned eighteen and I am worried about her, if she moves out, as she so often threatens when I try to reprimand her, then I will no longer have control over her. I found the perfect solution within the law, "if a young person is not seen mentally fit to be an adult at 18, the court can order her in custody of her parents until she is 21." When I told her she was livid, she told me she hated me and storms off to her room.