Other mistake: When the girls leave camp, if you look closely it is obvious that one of the girls (the one in the distance) is a double who does not look like Susan / Sharon at all.
Other mistake: After the camping trip, in the kitchen, Maggie McKendrick remarks that the camping trip ended early. (It did so because of the blowout with Vicki.) Later, Maggie tells Mitch that she had made extra food for him and Susan for the next evening (done before they got home), because she and Sharon would be leaving in the morning. This is inconsistent.
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.