How can I make my bedroom more unique and comfortable? ?

A mural would be very tasteful, not tacky, a divider between your half of the room and your sister's. If possible, make it an alcove kind of thing. Bring in chairs if possible, set up a conversation area w/ a coffee table between you and your sis. Think~what's your fave color. Reflect that in bright fabric, wall art, etc. Even the ceiling provides deco opportunities, for your wall art~ceiling art, maybe a canopy. Check this out for easy project.