Someone's Watching — Andrew Neiderman

Marty, age seventeen, and Judy, age fourteen, become stepbrother and stepsister when their parents marry. When Marty's father attempts to attack Judy, Marty beats him off. The pair flee to a nearby, seemingly deserted old resort in the Catskills, where they discover the owner still living as if the hotel were about to open. They participate in the old man's fantasy to keep themselves safe, carrying on a Romeo and Juliet romance, unaware they are constantly being watched….