So, this is my first chance to review this particular story and instead of starting at the begining, I'll just jump in right here...
First, this story immediately has a high standard to meet, considering who the authors are. I'll even say that I thought the very first chapter was a little shakey, but only because I hadn't seen the movie in a while and couldn't quite remember the particulars you were trying to mirror, plus, you made Elsa kind of a pathetic figure, which I did not enjoy at all.
From there, however, it has definately grabbed my attention and become a lot more fun. I thought the last two chapters were especially good; you wasted no time getting the girls preferences out in the open, and Elsa seems to already be a little smitten. I can't wait to see how this plays out - it will definately be one of the more inventive ways of dealing with a potential parent/child relationship.
Specific to this chapter, I thought the dialogue was spot on. You captured Doc perfectly, especially giving Anna the "Future Girl" moniker. I think I'll really like him, although I am a little appalled at his interpretation of the science here.
It was in fact an instantaneous effect. If her brother was never born, neither was she. The reason that Anna hasn't been effected by it here is that she is no longer in her own time stream, which no longer exists as she knows it. Her going back to the past has actually created a whole new time stream and there should be no paradox. Why does this matter? Well, if the story follows the current course, they find true love but Doc sends Anna back to the future and they only have the memories of what could have been. If he has the science wrong, we then are given a situation where they can find true love and live happily ever after in the past. As Fruipit knows, I'll always take the happy ending!
Great to see the two of you working together! I hope that it continues for another 50 chapters!