If your interest in films dealing with complex, forbidden, or taboo relationships comes from a place of artistic or psychological exploration (e.g., tragedies, psychological thrillers, or historical dramas that examine the destructive consequences of such dynamics), I can provide a list of acclaimed films that approach these themes critically, without romanticizing them.
Here are some incest-themed romantic films that explore complex relationships:
Victorian luxury, haunting ghosts, and velvet-drenched aesthetics.