The Sisters

Harriet, the older sister, is immaculately neat; Louise, the younger sister, is a slob. (think: ""The Odd Couple"".) They live together in a cliff-house in Maine. Not only does the domineering Harriet impose her tidy housecleaning habits on Louise, she tries to run her life too-- Harriet ""forbids"" Louise to date Paul Marchand. Possessive Harriet tells Louise she doesn't need men-- after all, they are sisters and will have each other forever.
One day, Louise lets Harriet fall off the cliff. Now Louise can do as she likes, and date Paul; (or so she thinks). At night, a woman like Harriet goes around compulsively doing housecleaning. One day, Paul comes to the front door, asking to see Louise. Even though Louise answers the intercom, it is in Harriet's voice that she tells Paul to go away. Louise has become her dead sister.
[in the character of Louise-- who adopted the habits of the person she killed-- I see a bit of Norman Bates, from Alfred Hitchcock's ""Psycho"" (1960).]

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