What's the title of this book?

Ok this was a Young Adult book I've read in seventh grade. I'm 22 now btw but it was really good.

It was about this girl who was living in the 1800s in the US and her brother got sick. Her mom told her that she was actualy living in the present and she was living in a 'biodome' like place. The mom helped her excape and the girl had to find someone to help her family before she gets killed by these assassins.

I know it's totally WTF but can someone tell me the title or the author at least, please?

That plot sounds like that stupid M. Night Shymalan movie that came out a couple years ago.

You know, the one where he gave away the ending ten minutes in by using modern speech pattern.