The changeover - Margaret Mahy

Carnegie medal-winning supernatural romance from Margaret Mahy.

The face in the mirror. From the moment she saw it, Laura Chant knwe that something dreadful was going to happen. It wasn't the first time she'd been forewarned. But never before had anything so terrible happened. The horrifyingly evil Carmody Braque touched and branded her little brother - and now Jacko was very ill, getting steadily worse.
There was only one way to save him. Laura had to change over: had to release her supernatural powers. And that meant joining forces with the extraordinary and enigmatic Sorenson Carlisle.