The most powerful scene was the ending, but not the part you think...
To me, it was the embrace and mutual screaming of the mother and daughter.
So much can be tied into that one shot for both characters. ***SPOILER*** While both seem to realize the horror of the double murder, they both seem to also finally surrender to their personal demons.
The crazed sister cries miserably, but not so much for what she just did, but for her succumbing to what she has become. You can almost feel her descending into insanity, and you can sense that the daughter knows she's descending, but can do nothing to abate her decline.
The mother, almost eerily, chooses to embrace the daughter that just killed her child, the daughter she could not love, and screams in horror, perhaps, in reflection of her failings as a mother, the indifference of her maternal efforts, the horrible relationship she had with her own mother. There's much more that can be interpreted there, so many directions you can go with it. It was a brilliant ending, executed to perfection by both actresses.
I'm a fan of competent writing and direction, and I'd love to hear the writer and directer's mindsets on that last shot. Incredible work.