Why don't you try doing a curtain wall. That way she has her side and you have yours, also it's pretty cheap to do. You can place book cases in between to divide the space as well. Here are some sites that might help:
http://images.g…
http://www.bran…
http://www.dvid…
http://www.cana…
http://www.luxu…
http://images.g…

thumbtack a sheet to the ceiling and let it hang down. that's what me and my older sister used to do

either some well hung opaque fabric to make a moving wall or how about a chinese room divider? They are made of wood frame and come in 3 or four panels hinged together. they look nice and you can always close it and pick it up to clean etc.

tack up a sheet to the wall to divide the rooms.
its inexpensive, and convenient.

build a wall!