Summer of Fear — Lois Duncan

What is it about lovely cousin Julia that fills Rachel with terror?

From the moment Julia arrived that summer, she seemed to seep into Rachel Bryant's life like a poison. Rachel knew she should feel sorry for her seventeen-year-old cousin. But why didn't Julia seem more grieved over her parents' recent deaths?

Everyone else was enchanted by Julia -- Rachel's parents, her brothers, her friends, and even her steady boyfriend. No one but Rachel seemed to notice Julia's frigid stare, her womanly body, the strange possessions she took pains to hide. Rachel's normally gentle little dog feared and loathed Julia from the start. What was it about her? What did she want with the Bryants? Soon all Rachel's questions were replaced by terror,--as she desperately tried to bridge the gap that had sprung up between herself and those she cared about most.