Demoniac (1958)

Gervais escapes from a German concentration camp and assumes the identity of a recently deceased fellow prisoner. Knowing that the dead man has been carrying on a romance by correspondence with Hélène, whom he has never seen, Gervais makes the acquaintance of the woman and they move in, but her sister Agnès dabbles in the black arts, which should be warning enough for Gervais to make himself scarce. Yet he sticks around, intrigued that the dead man's sister, Julia, refuses to blow the whistle on him.