Deep and Dark and Dangerous

Just before summer begins, 13-year-old Ali finds an odd photograph in the attic. She knows the two children in it are her mother, Claire, and her aunt Dulcie. But who's the third person, the one who's been torn out of the picture? Ali figures she'll find out while she's vacationing in Maine with Dulcie and her four-year-old daughter, Emma, in the house where Ali's mother's family used to spend summers.
All hopes for relaxation are quashed shortly after their arrival, though, when the girls meet Sissy, a kid who's mean and spiteful and a bad influence on Emma. Strangest of all, Sissy keeps talking about a girl named Teresa who drowned under mysterious circumstances back when Claire and Dulcie were kids, and whose body was never found. At first Ali thinks Sissy's just trying to scare her with a ghost story, but soon she discovers the real reason why Sissy is so angry.