Scream of Fear (1961)

Penny Appleby's parents were divorced when she was young and she grew up with her mother. After her mother's death, wheelchair-bound Penny accepts her father's invitation to live with him, but when she arrives, she learns that her father is away, though her stepmother, whom she's meeting for the first time, warmly welcomes her. On her first night there, Penny sees a light in the summerhouse; when she investigates, she sees her father sitting there--apparently dead. Others see nothing there and tell her she must have imagined it. She later sees his body in the library, then in the pool. She comes to believe that her stepmother and her father's friend, Doctor Pierre Gerard, are plotting to drive her insane. She gets a sympathetic ear from Bob, the chauffeur, who feels that there must be some rational explanation for what has been happening.