Humbug — Nina Bawden

Cora's stuck in a strange house with a girl who's out to get her--and no one believes her.

Cora was supposed to live with her grandparents while her mother and father were away. But Grandma had an accident, and Cora was sent next door to stay with "Aunt Sunday" and her daughter Angelica. Angel acts sweet and kind in public, but when the two girls are alone, she is vicious and spiteful. First she teases Cora, then she threatens her, and she keeps on tormenting her until Cora feels as if she's going crazy. Will Angel drive Cora over the edge?