*SPOILERS*
Even before watching the movie they already beat you over the head with the "twist" by the title and poster alone. So right from the start of the movie you have an idea that something bad is going to happen. Then the movie slowly, and I mean slowly, tries to get going by beating you over the head with the main characters mental instability, but it never really pulls you in to the point of caring for the characters. After watching the part where the daughter falls and he shoves his wife, you know that there is nothing left to watch. He shoves her in one direction and magically she appears in a completely different spot with everything OK. Her entire body language changes and so does the entire tone of her facial expressions. At that point you know you've reached the end of the movie and you can save yourself an hour plus of movie watching boredom.
The movie is so transparent in the "twist" that there is no point to even watch it. If you watched it and it took you longer than that point to figure out what was happening, I'm sorry for your loss.