Trivia: Hayley Mills mom accidentally mistook Hayley Mills stunt double for Hayley. Hayley's mom tapped the double on her head and said "Hayley come on."
Trivia: For the scene when the camp girls are following the twins to the cabin Miss Inch was putting them in, the director wanted the girls to march like soldiers. The girls could not march very well, so he just decided to leave it as it was. The whistling the girls do is not the girls' whistling--its a recording of the march from Bridge on the River Kwai.
Trivia: Hayley Mills had one stunt double that can be seen in the movie when one of the girls has her back to the camera or when Hayley is doing something she can't do by herself. The stunt double girl received a Duckster award for the best unseen performance by an actress - only three have ever been awarded.
Trivia: When Susan is talking to her friends at the lunch table after seeing Sharon for the first time, if you look closely, you can see the double for Haley Mills behind Susan's table.
Trivia: Hayley Mills' father, John, makes an uncredited cameo as Mitch's golf caddy.
Answer: None. It's total fiction made up solely for the purpose of the movie. Even for a movie, it's far beyond the "suspension of disbelief" that siblings would ever be divided up between the two parents, and neither would have no contact with them, much less be prevented from knowing they had a brother or sister.
raywest