The Midnight Visitor (Judy Bolton Mysteries)

When Judy and Peter become stranded in an abandoned house during a storm, the last thing they expect is to meet a ghost. The ghost turns out to be a girl named Sally who insists that she is being plagued by ghosts. While Judy and Peter hardly believe the girl, they do like her and want to help her. Judy takes her home, only to have her disappear during a party. Later, Judy and Peter learn that Sally's full name is Sally Vincent, of the crooked Vincent family. Sally is fortunately not like the rest of the family. She is to receive an inheritance in a will but the rest of the family is contesting it. Peter agrees to be Sally's attorney even though fighting against the Vincent family could prove to be dangerous. As the case unfolds in court, Judy makes a shocking discovery that will mean everything to Sally and will thwart the plans of the rest of the Vincent family.