Deep and dark and dangerous

Ali goes on vacation in maine with her aunt dulcie, and her baby cousin emma. soon emma and ali meet a sassy girl named sissy, who is a bad influence on emma. Sissy keeps talking about a girl named teresa, who drowned back when claire, Ali's mom, and dulcie were little kids, and whose body was never found. At first Ali thinks that sissy is just trying to scare her and emma, but soon figures out why Sissy is so angry.