An Explanation for those who don't understand. (spoilers)

I can honestly say that this would of been a blockbuster hit if they hadn't tried to ride the (already failing) bandwagon of Butterfly Effect. Although the story might not seem well thought out at first, you simply need to think about the brother and sisters abilities and the times in which they use them. I'll start from the beginning.

Sam originally goes back in time to save his sister from dying, which in turn kills his parents. Although the parents escape the first time, they seem to die for some unknown reason; even the question of how the fire started seems unexplained unless you think about what you saw and what was said at the end of the movie. 1)His sister can jump just like he can 2)His daughter has an infatuation with fire 3)Sam was inside the house holding the door shut, trying to kill his sister instead of outside with a ladder. Why would Sam be outside in the first place and inside secondly?

Originally Sams future self set the fire to kill his sister. This is why he was outside when Sam jumps back in time to save his sister, however he has no idea this happened because as he returns to the future each time he gets more and more confused about certain events. Even his future serial killing sister tells him that she has explained the situation multiple times.

Everything goes fine the first time, his sister dies, and he saves his parents from getting killed in the fire. However, as soon as Sam jumps back to his future self, his past self forgets starting the fire and this is where the problem starts.

He once again goes back in time, however he jumps to the time after the fire starts, so he wouldn't know he started it in the first place. He ends up saving his sister and his parents die in the fire trying to find their children. The house burns down to the ground and his sister is saved.

As the story progresses his sister continues to jump through time with him and slaughter the women who are trying to steal him away from her. She continues to cover up evidence that would implicate her. This is what makes most of the changes from Sams jump. Sam is doing very little to change his future, but his sister is constantly stepping on butterflies and making his situation worse.

So we now get to the end of the movie. Sam drowns himself in order to jump back in time. Sam is effectively dead, there is no future Sam anymore. His consciousness is stuck in the Sam that went back to the warehouse and got stuck in the beartrap. This is why he is able to use his ability to travel back in time again. Knowing that her brother is about to go back in time and try to kill her, she follows him back. Expecting him to be home when the fire started she awakes in her past self and runs for the door, only to be met by Sam who is holding the door shut.

You might ask why he is now inside instead of outside...

This might get confusing at this point but try to follow. The original future Sam goes back in time to save his parents and the fire has already started. He didn't start it, his future daughter and sister did(as I explain in the last paragraph) You know this is the future Sister behind the door because she clearly yells at Sam for holding the door shut, even though she never see's or hears who is behind the door; until he tells her he will always love her.

Jenna is never shown to be engulfed by flames, or pass out or anything of the sort. This is a stretch of my imagination and I don't quite have the part about Sams daughter down but...Knowing their abilities it isn't too much of a stretch for Jenna to wake back up in the future before her past self dies, and then go back into the past once again before the fire even starts. In the future of the birthday party, Sam is confused and knows nothing about what is going on.

I honestly think that Jenna returned to the past and stopped her brother from starting the fire. She still exists along with her brother and parents, however Sams consciousness would still wake up in the time of his future bear-trap self. He knows nothing about Jenna and thinks she is dead, however she is not. Knowing this Jenna lives her life normally up until the time of the birthday party. She infects the daughter with her propaganda and shows her how to jump back in time. She tells the daughter to jump back in time to start the fire while she is upstairs, effectively killing herself because she knows after the point of the birthday; they could never be together. She leaves her influence in the child and essentially still lives on to be with her brother.