Agatha Christie
Agatha Christie wrote a book called "The Moving Finger" which was published in 1942. Its about poison pen letters sent to people in a small town. The first two victims we meet are a brother and sister who have just arrived in town. They are accused of being lovers posing as brother and sister. Then they find others have been receiving them for some time before their arrival.
This made up an episode on PBS "Miss Marple Mysteries" a couple of years ago.
The book came out the same time the movie was being produced in France. Could one have influenced the other or maybe both were influenced by the real crime which happened in France ca 1922?