My Sister, My Love (1969)

Diana's marriage has ended in divorce, so she goes to stay for a while with her younger sister Martha, whom she hasn't seen in ages. However, Diana is actually there to try to rekindle the passionate incestuous relationship that she and her sister once had until Martha decided to end it and leave Diana for good. Diana accepted it at the time, but after she got married she realized she could never love anyone like she loves Martha and she will never be happy without her. But Martha is now married to a wealthy older man and enjoys her Diana-free married life, although she does cheat on her husband occasionally--but he doesn't mind. Diana, mad with longing and lust for her sister, tries everything to convince her to come back to her, but Martha would rather die.