Dogtooth (2009, directed by Giorgos Lanthimos)
Such a great premise, wasted. Ah well.
A couple, living in an estate/compound completely cut off from the outside world, have raised their three children to near adulthood in complete isolation. They've taught them the wrong words for things (if the movie becomes a hit back in its native Greece I fully expect the phrase "Lick my keyboard" to become big) and constructed an elaborate mythology about the outside world, the world father bravely heads out into every morning to go to work despite its killer house cats and other dangers.
At the factory where father works, he's begun bribing a female security guard to come back to the house with him periodically in order to, umm, service his son. This causes problems, as Christine would much prefer to have her keyboard licked by the eldest daughter, so she begins bribing eldest daughter to service her. Eldest daughter then begins bribing younger daughter likewise, although she's unclear on which parts of her are supposed to be licked, and the parents' twisted version of paradise is threatened.
I've heard more than one person describe the film as a feature-length version of Steve Martin's 'mambo dogface to the banana patch' routine, and sadly that's not far from the truth. There are some funny bits (I particularly liked the idea of passing planes occasionally falling into the garden as toys for the kids, or the pool randomly spawning fish which the father then goes after with snorkel and spear gun) but when the film tries to be shocking it falls completely flat. If you don't figure out that there's going to be incest somewhere along the way after watching the first five minutes of this movie, you probably shouldn't be watching anything more narratively difficult than Transformers.
The big problem though is that the universe just doesn't make a lot of sense. The movie is apparently set in the real world, which begs the question of how the parents have gotten away with it for so long. These are smart, curious kids, and it's simply not plausible that one of them wouldn't have started sneaking outside the gates before this, even if just to the other side of the road.
There's a good movie in this premise somewhere. Dogtooth, however, isn't it.