Charmed Life — Diana Wynne Jones

SURELY GWENDOLYN CHANT WAS DESTINED FOR GREAT THINGS.

Her magic was strong. Strong enough to gain the respect of all the witches and warlocks on Coven Street. Strong enough to draw the attention of the awesome Chrestomanci. Strong enough to get Gwendolyn and her brother, Cat, transported to Chrestomanci's castle where Gwendolyn planned to learn even more powerful magic from the great enchanter.

But the castle was not as Cat and Gwendolyn expected. Cat watched Gwendolyn rebel as Chrestomanci ignored her talents -- until she had unleashed a magic so powerful that she couldn't control it. A fearsome magic so strong their whole world would reel under the force of it.