Conrad's Fate by Diana Wynne Jones, a Chrestomanci novel

When Anthea Tesdinic turns her back on her family and leaves for university, her brother Conrad is left at the mercy of his magician uncle Alfred and his distracted mother. Conrad helps out at the bookstore his father and uncle founded in the English alps and hopes to attend high school until Uncle Alfred says he has bad karma, likely due to something he did in a previous life.