Out of Season (1975)

In this tense story of an unusual romantic triangle, middle-aged Ann and her teenage daughter Joanna manage a failing hotel on an island off the British coast. One day, the hotel receives an unexpected guest, a man named Joe. He and Ann had an affair years ago, and it's suggested (though never stated) that Joanna could be their child. They soon renew their relationship, but Ann discovers that she has a rival for Joe's affections when Joanna makes her interest in the handsome older man clear.