The ending makes no sense - ! MAJOR SPOILERS !
The ending makes no sense for two reasons :
--) The whole reason Sam never went back in time to try to save his girlfriend a second time was because according to his friend Goldburg he couldn't go back to the same point in time without frying his brain. So he had to go to the time of later murders to figure out who the killer was. Now how the hell did he manage to get back to the time his parents' house burnt a second time? Didn't he say he'd already gone back through time to save his sister? Shouldn't it have driven him insane rather than turning everything to normal?
--) Why would his sister become a nutjob if her parents stayed alive? Wouldn't saving both his parents and his sister have solved that problem as well?
Overall, "The Butterfly Effect 3: Revelations" is far superior to the second film but the ending pretty much desroys the film.