Earwig and the witch / Jones, Diana Wynne

Earwig enjoys life at St. Morwald's, where she manages to make everyone do her bidding, until she is taken in by foster parents--who are actually a witch and a demon--and struggles to turn the situation to her advantage.

Although an orphan, Earwig has always enjoyed living at St. Morwald's, where she manages to make everyone do her bidding, but when she is taken in by foster parents who are actually a witch and a demon, she has a hard time trying to turn the situation to her advantage.