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.
Character mistake: Mitch and Hecky are experienced ranchers - so why, when they are all camping, would they leave all the food out in the open within easy reach of bears and other wildlife?
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.