How did he undo what he changed and other flaws

Rating: 6/10 (27 June 2015)

I like movies about time travel, but this is full of flaws (more than others).
First, how did he know that he can take other persons with him? Did I miss something? And when he took his sister to "her" past and she changed it, they jumped back/forward into the present and both new immediately what changed. His wife asked him what happened in the past but how could she as the old time line does not exist anymore and in the new time line he had no need to travel back.
Then when he realized that his daughter turned into a boy, how did he undo what had changed? The new time line was already in place, so would he take his now happy sister to the past again? And as she knows the old and the new time line she would act the same way as in the first time travel. Or did he return on his own? Then he would have met himself. Or did the travel back replace him with his other self and he strangely - in the eyes of his sister - changes his mind and just says nothing?
As another writer mentioned, if he always comes out in his cupboard what happens with him in that past? Does he just disappear? And what would happen if that cupboard didn't exist in that past?
When he visits his father in the past in his living room, why did his father know that he time traveled? Didn't he ever meet his father in that time line the "normal" way?
When he visits his father for the last time we can hear his father and him(?!) play table tennis as he just exits the cupboard and then the camera jumps to both playing. So he existed twice for some seconds?
In this scene his father acts as if he would never see his son again. But that is not true, because Tim in that time line lives his normal life. Does his dad ever tell him that he will visit him in the future? Obviously not, because otherwise Tim would know that he visits his father before he does it.
That is the big problem of (impossible) time travel. You change the memories of yourself when you tell someone in the past about you and he tells it to your other self. So you must have that information BEFORE you even change the past. The same thing as killing your grand pa.
The film "Back to the future" handled this paradox in a more elegant way as the people really existed more than once in that time lines and so they could change their own behavior.