Summer of Fear, Paperback/Lois Duncan

From the moment Rachel's family takes in her orphaned cousin Julia, strange things start to happen. Rachel grows suspicious but soon finds herself alienated from her own life. Julia seems to have enchanted everyone to turn against her, leaving Rachel on her own to try and prove that Julia is a witch. One thing about Julia is certain-she is not who she says she is, and Rachel's family is in grave danger.