How can I make a shared room liveable?

Hi. My sister and I might have to share a room after having our own rooms when my family (my parents, brother, sister, and me) move in a few months. I wanted to find some suggestions as to make the move bearable. We are moving to a 3 bedroom house from a 4 bedroom house (one of the rooms was a study). The top floor is the master bedroom with a bathroom and a walk in closet. The second floor has 2 bedrooms, a bathroom, and some closets. The ground floor is the living room, dining room, kitchen and some more closets. My mom said to make us more comfortable my sister and i can take the master bedroom. How would I divide the room to make it more comfortable. The options I thought of was: a dry wall (but there's only one window) or using a curtain somehow. Any suggestions would be appreciated. Thanks in advance.

How big is the room? Perhaps book cases back to back separating your beds or one of those 3 panel shades that people use?