Surprised so many people missed the real twist (SPOILERS)
I see so many people comment about the twist in the message board and also in reviews but the miss the real twist of the movie.
The real twist is Casey also has SPLIT personality and displays superhuman body chemistry change at the end of the movie (Skin deformations at the end). While Kevin develops split personalities to deal with his mother's physical abuse, Casey was sexually abuse by her uncle/guardian and retreats into her psyche to draw superhuman strength at the end.
The thesis of the movie is people who are abuse retreat to deep into their psyche and tap into a resource that is superhuman. Shymalan suggests that there might be lot of people with superhuman talent and suggests what can't kill you can only make you stronger!
Interesting question is did Bruce Willis also suffer from some abuse before he became superhuman?? Shymalan seems to suggest that as per the split-mythology there are positive superhumans like Bruce and negative superhumans likes Kevin.
Anyway, based on audience reaction looks like the "twist" didn't work as Shymalan expected. Not sure he meant twist to be this subtle. This is going down more like the twist in "Life of Pi"....."Why didn't Richard Parker(tiger) didn't turn and look at him"
Interesting to see audience enjoy this movie even if they missed the twist. For me, it is an okay movie.