The Black Cat's Clue — Margaret Sutton

Fifteen year old Holly Potter has returned to visit her Uncle David Potter during a family reunion he planned at his home, only to learn that he was recently killed in a train wreck in Florida. Holly is afraid to face her relatives alone so she convinces Judy to pretend to be her redheaded older sister Doris. Judy plans to masquerade as Doris until she and Holly can learn why Holly's relatives are searching Uncle David's house and whether Uncle David is really the person who planned the family reunion. Soon Judy has more to investigate than scheming relatives. Is Uncle David's ghost haunting his house, or is Uncle David still alive? Judy has several thrilling adventures in the Potter home until she learns the answer.