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 ★
It was during the 1960s, the courts had no way of forcing parents to share children. They could have very easily just stayed away from each other out of the view of the judicial system.
This is what I always assumed as well. That this wasn't decided by court, the parents decided this on their own and did not bring it up to the court.
There has actually been a history of separating identical twins as babies, as there has been a fascination in studying what ways they'd be alike, and how they'd be different. During this time period, there were even agencies that would pay women who gave birth to identical twins to give them up for adoption, and have them be adopted in separate families. In today's world, this would not happen, but I wouldn't put it past a judge back in the 1960s.