Trying to find a book I read in 2009?

Hi,

I read a book in Grade 9 English that I'd really like to find but I can only remember the bare minimum of the details.

Summary of what I can remember:

There was a sister and a brother and when they go to sleep they travel through time (though from what I remember their bodies don't leave the present time) and the boys sister gets trapped in the future in her mind or whatever and won't wake up so the brother tries to find a way to get to her.

In the meantime the sister meets this a guy and in the time there is some kind of dictator that people are trying to overthrow and the sister is suspected as being from thee enemys side (i think for that last part not too sure about the enemy part). In the end the brother finds her and goes back to the present time but this was a great deal of time later for the sister and she was pregnant, maybe married? etc etc.

Other things i can remember: In the present time they were being studied by someone while they were sleeping to try and figure out why/what was happening and I'm pretty sure the only reason the sister decided to go back to the present time was because she was injured.

You can see why with my very little memory of the book I'm struggling to find it

Thanks