The Time of the Ghost
Diana Wynne Jones1981

Can a ghost from the future save a life in the past? A chilling tale of dark forces and revenge! The ghost turns up one summer day, alone in a world she once knew, among people who were once her family. She knows she is one of four sisters, but which one? She can be sure of only one thing -- that there's been an accident. As she struggles to find her identity, she becomes aware of a malevolent force stirring around her. Something terrible is about to happen. One of the sisters will die -- unless the ghost can use the future to reshape the past. But how can she warn them, when they don't even know she exists?