Sharing a room with a sibling, or two, can work and be more or less peaceful. I have found that the most important thing is to establish what belongs to each child personally and what they have to share.
Let me use my girls’ room as an example: My three girls share a room that is roughly 12’ x13’ with a small closet. In the room is a bunk bed that is a twin over a queen, a desk, two bookcases, a play kitchen, and a night stand.
The beds: Each child has their own twin size quilt and two pillows. I left one tag on each pillow and wrote their name on it so there are no questions when we change the sheets. Having their own quilt means the two girls sharing a queen bed aren’t fighting over covers. Making the bed looks weird but I would rather have that than a fight every night. On Saturday we change the sheets and pillowcases and I let them mix and match whatever sheets we have. Each child also has a bag that hangs over their headboard to put books and a flashlight in because they are allowed to read in bed. Each person’s bed or side of bed is their own personal space that others are not allowed in. Sometimes I have to persuade girl3 to leave her big sister alone but it happens rarely.
The bookcases: We are a reading family and have quite a few books. In the girls’ room is the books that the girls want to read and/or are their current reading level. Right now the board books are on the bottom for girl3 to easily get to with the picture books above those. Girl12 and girl9 are both into chapter books so those are together by series where they can reach them. On the top shelf is where the two big girls keep some pretty things that they don’t want little girl to break like a paper fan, a music box, and some picture frames. The other bookshelf has their 18” dolls and furniture on it. By taking out one shelf at the top, the four dolls we have (two from my childhood and one each for the big girls) hang out on their stands quite comfortably. The lower shelves contain most of the furniture the dolls have (I have my old stuff on the top of the closet so girl3 can’t get it) including a kitchen, two desks, and a table and chair set. The doll accessories are in totes on top of the bookshelf and the clothes hang in the girls‘ closet. Each girls’ doll is their own but they share the clothes and accessories.
The desk: This is more of a craft table and the two big girls have it divided with each girl having a pencil cup and their current craft stuff out.
Each child also has a ‘special papers‘ tote in their room. In this tote is any papers that they want to keep. They can only keep what fits in the box.
Giving each child their own space and clearly defining what belongs to them, what belongs to a sibling, and what they have to share means that fights over stuff are rare and usually taken care of with the question, “who does it belong to?”