Deep and Dark and Dangerous by Mary Downing Hahn

Ali's mother has finally relented and is allowing her to spend her summer in Maine babysitting her four-year-old cousin Emma while her Aunt Dulcie paints. Her summer does not go as planned, however, when Emma meets Sissy, a mean and spiteful girl who is not a good influence on Emma. Things become even stranger, however, when Ali learns that a friend of her mother and Aunt drowned under mysterious circumstances when they were kids. Ali sets out to learn more about Sissy and Teresa, but what she uncovers she never imagined would be possible.
Hahn is the master of chilling ghost stories. This one has the feel of Wait Til Helen Comes with its mystery and horror elements. A quick read, this story will engage readers and keep them hooked until the very end.