The Buttercup Chain

Born to twin sisters on the same day in London, France and Margaret are more like brother and sister than cousins. The close relationship between the cousins is suspended when Margaret is sent abroad for schooling. Now a young woman, Margaret returns to London and resumes her intimate connection with France. Fearing that their relationship could become incestuous, France decides that he and Margaret should take lovers, and chooses Fred, a Swedish architectural student, to be his cousin's lover. (imdb)