Incest, a strong theme in this movie
The major theme of this movie is incest IMHO. The father has a strange relationship with his daughter (almost romantic). You see the way the mother is looking at them both when he gives his daughter the jewelry box...it's a really weird moment; and it's hard to read what her mother is thinking, personally I think it's jealousy, but it could also be fear that the father will abuse the daughter.
The mother married her uncle (the kid's father) and got pregnant to him out of wedlock.
There's also the scene where the mother is removing her blouse for the whipping and you can see her father's face twitch as she reveals her body. To me this suggested that he was probably sexually attracted to her and had probably sexually abused her as a child.
The whipping alone suggests sado-masochism and sexual depravity.
The mother, has become hardened over the years, as the father has probably shown no sexual interest in her for most of their marriage, because his daughter was the focus of his sexual desire. The mother resents the daughter for this and takes out her rage on her and her children.
I also understand in the book that the brother and sister engage in incest whilst they are locked in the attic.
It's a very interesting story/film.