Deep and Dark and Dangerous by Mary Downing Hahn
Just before summer begins, Ali finds an old photo in the attic that shows two children she recognizes but the third one has been cut out. Who is the third person, torn out of the photo, she desperately wishes to find out. When she asks her mother she just avoids the answer. Something is going on Ali is curious what.
She takes a job babysitting for her Aunt just so she can go up to the old house where her Mom and Aunt used to vacation as children. When Ali and her young cousin Emma meet a sassy young girl named Sissy everything starts to go wrong. Sissy keeps talking about a young girl who drowned named Teresa. Who is Teresa? Why did she drown? What does Sissy know about it? Sissy is mean and spiteful always playing tricks on Ali and Emma. She is just waiting, to show them where Teresa's is, to reveal what actually happened.
Read the book to find out if Ali is able to figure it out and solve the missing photo.