Dad is too busy to engage his daughter because he's too damn busy repairing the Sybian. Also, the older sister is pretty hot despite the abundant growth of pubic hair she is probably sporting that was common in the 70s.
And Susie grew up to be... nobody of any relevance, because she embraced a religion that had no tolerance for individuality except in unobtrusive and minor cases, most particularly in women and girls. And now you know... the rest of the story. GOOD DAY!