The Ragwitch — Garth Nix

Beware the Ragwitch...
As Julia and her brother, Paul, explore a deserted beach on Australia's northern coast, they find an enormous nest filled with the largest, strangest black feathers -- feathers that neither of them can identify.

But there is something else in the nest, hidden among the feathers -- a small rag doll. When Julia cleans the doll off, it is more beautiful than any she has ever seen. Almost hypnotizingly beautiful.

Indeed, this is no ordinary doll -- as Julia discovers when the doll absorbs Julia's spirit and takes over her body. In order to save his sister from the evil Ragwitch, Paul follows them on a harrowing journey across time into another dimension! There he must battle the Ragwitch and her forces of evil. If not, Julia will be lost... forever.