Hi, Ezra...it's awfully nice of you to be concerned about your cousin. You're a very caring and sensible young man, from what you post.
I think that since you and your cousin are so close, it will straighten itself out eventually, as the two of you learn more about each other's special routines, creature comforts, pet peeves, etc; but if problems do arise, perhaps it might be wise to set some boundaries/guidelines with your parents or another adult you both trust. Sometimes folks who are great friends can have difficulties actually being roommates.
Anyway, to answer your question....yes, I have had experience sharing a room. When I'd go to my mom and stepdad's house every other week as a kid, I would share a room with my two younger half-sisters, Pandy and Birdgirl. It started out with Pandy and I actually sleeping in the same bed while Birdgirl slept in a crib (they were both pretty little back then) and then Pandy and I got bunk beds. Pandy slept on the lower bunk, and I slept on top, since I was older....for safety reasons. I really wouldn't have cared which bunk I had. Birdgirl still had her crib.
Then, Birdgirl grew out of her crib, and then she and Pandy shared the lower bunk, while I slept on top. Then, my mom and stepdad moved, and then Pandy and Birdgirl shared a room (and bunk beds) and I had my own room (when I came to spend time with them, per the child custody agreement).
I didn't really mind having to share a room with Pandy and Birdgirl...I actually would feel a sense of security knowing that both of them were in the same room as me. It was just something I became used to. I didn't like it when they'd get into my stuff though....I just learned to keep my favorite things in high places where they couldn't reach, or carry them around with me at all times.
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I wish Sterling Holloway narrated my life.
"IT'S NOT FAIR!" "Life isn't fair, Calvin." "I know, but why isn't it ever unfair in MY favor?" ~ from Calvin and Hobbes