Charmed Life by Diana Wynne Jones

Cat has always clung to his sister Gwendolen . . . especially after their parents die and they are orphaned.  When Gwendolen gets invited to Chrestomanci Castle to further her magical training, Cat goes along.  Gwendolen is sure the Chrestomanci will be impressed with her magical abilities.  When he ignores her and refuses to teach her magic, she vows vengeance.  She casts a new spell each day, each one more terrible than the one before.  Then she decides she has had enough and casts the biggest spell of all.  She disappears to another world and pulls her "twin" to take her place.  Now Cat must help the replacement while fixing all the trouble Gwendolen has caused.

Diana Wynne Jones was an inspiration to Harry Potter author J.K. Rowling.  I can tell why.  Her books are wonderful.  I would highly recommend this series to anyone looking for a new fantasy book to read.