The Stepsister

The Stepsister is a horror novel by R. L. Stine, published in 1990 and filed by the library under the Fear Street collection.

Emily's home is rearranged when her new stepsister Jessie moves in. Jessie claims Emily's room, helps herself to her clothes and tells lies about her to the rest of the family, and Emily's attempts to like her get harder by the week. Then Emily reads Jessie's diary and learns something she was never meant to see, and she begins to wonder whether her stepsister is capable of murder. She takes what she has found to her parents, and they do not believe a word of it, which leaves her alone in the house with somebody she is afraid of.

The house the two families share is small enough that Emily cannot avoid her stepsister, even when she wants to. The novel is one of a long line of Fear Street horror stories.