Hahahahahahaha! Have you ever tried sharing a car with your sister? We had some beautiful battle royals over our schedules, mud on the floor mats, ash in the tray, books in the trunk, and leaving it on empty. We love each other, and car sharing made us want to kill each other. :p
That sounds similar to stories I've heard about "best friends" going on vacation together and sharing a motor home or a camper. It was great fun to listen to people talk about their "best friends" weird behavior. I'm sure the other parties had their stories as well.