I've never played these games or ever seen any play throughs for the story but this little fan made comic has put an idea in my head. If these games were to ever get a remake perhaps they could add milf routes for the mothers of the twins.

Looking at this page it gives me the impression of a pretty young mother who is either raising her daughters alone or the father is just away most of the time. I've seen eroge in the past with mothers or big sisters getting in on the action while the player character does the deed with the daughter/little sister.