That's a charming book where Cat discovers he is more or less on his own even if he tries to support his talented sister. But he can't help getting in trouble. I wish I read it as a child, but it's a book that's magical for grown-ups. 
 So Cat and his sister Gwendolen are orphans at the beginning of the book but Gwendolen manages to get them cared by the Chrestomanci a powerful enchanter. Except the two orphans can't get along with him and his family, especially because of Gwendolen's behavior.