Saw the end coming a mile away

I first saw this movie years ago when it first came out. I remember Tamsin telling Mona about her sister dying and thinking that she was lying about it. Then, as the movie progresses, I sensed it and that feeling didn't go away. There was something about Tamsin and her behavior that was off from the beginning. The relationship was more from the heart and genuine from Mona. For Tamsin it was power, manipulation, and a game.