Anna *contains spoilers*
I just saw this movie for the first time this evening. I found it extremely painful to watch, but yet couldn't turn it off.
I think Anna set up this relationship of herself entwined between father & son as a way of replaying her relationship with her brother when she was a child and his resulting suicide.
Since she chose these two related characters, and knew very well the dangers that were involved, she was putting herself in a position of regaining power; the very power she lost in being a victim of incest, and the power she lost with her brother committing suicide because of her relationship with a boyfriend.
Her brother did the two very worst things to her that could be done - she was a victim of incest and then, when she found a boyfriend, he killed himself to get even with her. Through setting up this relationship with father & son; she gets to replay the event - only this time she calls the shots. The fiances death liberates her. The son never stood a chance in this relationship, her intention all along was to turn him into her brother and see him die on her terms.
Abusive relationships have a way of worming their way into the psyche of people and it is very difficult to break that pattern. This movie is one of the best examples of the cycles of abuse that I can think of.