Sister My Sister, an other movie about the Papin sisters. A few years ago I watched Murderous Maids, so I was very familiar with the story. Like that film, Sister My Sister is claustrophobic and slow.
Although the incestuous relationship is kind of fascinating, I felt that the intense, murderous, insane rage was somehow not quite developed. They gauged the eyes out of their employer and violently murdered her and her daughter. Yet I didn't feel a build up to this. Yes, they were intense and crazy and weird. But the violence didn't seem to fit with the rest of the mood, and I felt more perplexed at the end than anything else.