 According to John Wayne (I) (qv) in a 1974 interview,  John Ford (I) (qv) hinted throughout the movie that Ethan had had  an affair with his brothers wife, and was possibly the father of  Lucy and Debbie. This meant Ethans thirst for vengeance stemmed not  from the murder of his brother, but of the woman Ethan had loved.  This was so subtle that many viewers at the time missed it  altogether.