[TOMT] [book] Book about a girl who has a brother who wants to be a level 20 wizard

This is a book I read when I was young back in the 90's, although it was probably written in the 80's. The story was about a teenage girl but in the background of the story was her brother who was like a loner weirdo who played Dungeons and Dragons. Towards the end of the book he either wants to kill her and the rest of the family, or he tries to save them from the actual bad guy of the novel?? and says he's doing it in order to become a level 20 wizard.

I don't think the book was fantasy, it was definitely like a thriller type story. Thanks for any help!