Some theories / observations
At one point in the movie, we see the father leaving for work while his wife and children are standing in the garden, watching him until he's gone. At the end of the movie, after the eldest daughter has escaped, the two other siblings are notably absent from this morning ritual, with only his wife bidding him goodbye. Maybe a slight sign of "resistance" on the other siblings' part and a hint that they might soon attempt to escape as well?
I also loved the scene with the dog trainer and how his speech about the treatment of dogs mirrored the parents' treatment of their children.
And as for the "other brother" who's often been discussed here and whether he really existed or not: Maybe he was an older son of the couple and something happened to him, he might have had an accident or been otherwise hurt/killed, before the other kids were born. This could explain the parents' fear and paranoia of the outside world: they don't want anything like that happening to their other children, so they keep them sheltered.
“It is no measure of health to be well-adjusted to a profoundly sick society.”