Big part of the plot people seem to be forgetting *SPOILERS*
There is a huge thread debating the ending of the movie and what happened to the elder daughter, but I think the bigger question is what happened to the "missing" son?
Clearly from the unspoken words between husband and wife, you can tell that at some point the "other" brother escaped, thus making the elder daughter's escape the second time that one of the kids has run off. I think the meaning behind this (among other things) is that you can't destroy the human spirit, and that no matter how hard the father tries to keep the kids isolated, the outside world always comes creeping in. Essentially I view this as saying that it really isn't the outside world you should be worried about, but more the people and things closest to you. If you look at statistics on violent crimes, it tends to be the case in most countries in the world that they are committed on a person by a person close to them. This move demonstrates a type of "social violence" whereby the father keeps the kids through harsh punishments and "rewards" that aren't really rewards at all. In the end, the father is so obsessed with "protecting" his family that he becomes the biggest threat to them. The crazy little world he has created for his kids is actually much more dangerous than the outside world he is trying to protect them from.
Regardless, as the beginning of my post stated, I wondered what happened to the mystery brother. Did he exist at all? The mother's sorrow seems to express that another son escaped at some point. What must have happened to him if he did exist?
As for the elder daughter, if the movie were interpreted literally then she suffocated from lack of oxygen. On the other hand, if it's interpreted symbolically, then I'd like to believe she "got out" which was all she wanted in the end, and if she died she was still happier than being stuck at the house.