What the... ? (yeah spoilers)

Can someone explain wth happened, under what theory the movie isn't just totally jerking your chain for some effect?

So the younger brother /says/ he sees the older push her, fine; then, he remembers her just falling. ???? Is it just me or is this a crap movie with some "you don't really know what happened, the interpretation is up to the audience, it's deep, man" hot sauce thrown in to cover the taste?