In just my own lifetime's observations I think there are many many ways in which Cindy and Keith could have had and continued a forbidden incest (even unintentionally) eventually bearing both Ryder and Molly as incest children. The film gives very little background on Cindy and Don’s marriage; maybe Don thought Ryder was his. And another possibility could be a later marriage problem between Don and Cindy at some point, prompting a separation of a year or so when Ryder was young (say 7 or 8). Cindy could have returned home to her mother, and in her vulnerable state her incestuous relationship with her brother reignited.
But, yes, you’re right, there are just so many possible sordid and illicit plots implied by this film, it would be very interesting if the original screen playwright would answer all these questions and reveal exactly what his whole plot was behind the film, if there even was one. Maybe the open cliffhanger ending was meant to leave it entirely up to audience imagination.