Heartstones — Ruth Rendell

Sixteen-year-old Elvira's mother is dead. Elvira is sad, of course, but not so sad as her younger sister Spinny. Spinny is afraid their father, Luke, will be heartbroken, but Elvira knows better -- after all, Luke has her to take her mother's place. But then Luke brings home a pretty young woman and introduces her as his fiancee, and Elvira decides that she will stop at nothing to stop her father's marriage . . .