Guardian Award 1977.
Cat and Gwendolen were orphans, and when an old friend of their father's, the mysterious Chrestomanci, came visiting and offered to adopt them, Gwendolen, who was already studying Advanced Magic, knew that she was destined for Great Things - for hadn't the Fortune-Teller predicted that one day she'd rule the world?
Cat, who had never been able to manage turning brass buttons to gold, wasn't so sure he was going to enjoy his new life. But then what he thought didn't count for much. Ancient and turreted, Chrestomanci Castle was every bit as grand as Gwendolen had hoped. But there were disappointments. No butler opened the door to her, no lavish banquets were given in her honour.
Instead, she and Cat had to do lessons in the schoolroom with Chrestomanci's two children, and worse of all she was forbidden to practice magic except under supervision. In fact, she wasn't being treated with the respect she deserved, and she very quickly decided to do something about that.
It was difficult for Cat, who clung to his unpredictable sister as the only familiar person left in his life, especially when he guessed that Gwendolen was planning to go to far, and he didn't see how he could stop her. He didn't realise, though, quite how ambitious Gwendolen's ultimate plan was, nor how closely he himself was involved in it.
Magic is an everyday affair in the charmed world of this story, and you never know who may turn out to be a witch or enchanter. Like every book by Diana Wynne Jones, it bubbles with invention, and as the story unfolds the people in it grow and change with all the surprisingness of real life.