I wanted to parallel Cassie's difficult relationship with her mother but I couldn't find a way in that I liked and I think you've just given me a great idea that her mother is/was famous and close to his family, which could account for why he went to live with them and not someone else. Charlie has a dry, sarcastic sense of humor so they could have made a fun team. How old is he now? They could be close in age, making him the sibling that she always wished for.