I understand that some kids don't get along. Mine fight sometimes, but usually they are good friends.
I have a DS 18 and a DD 14 and they still share a bed when we are on vacation. I don't see anything innappropriate about it. I mean, in a pinch I could sleep with my brother (if I had to) and we are both in our 40s. And that's exactly what we'd be doing - sleeping. Mind you, it's not that I'd WANT to exactly, but I'd never feel it was inappropriate. Would it still be inappropriate if they were in sleeping bags on the same bed? Why is it okay with same sex siblings or with a parent okay, but opposite sex siblings not? I'm puzzled by this.

Mine never snuggle up in each other's arms or anything like that. They fall into bed and crash.
My kids have never complained. They grumble about the other sibling stealing covers, etc., but I do the same sometimes with DH. They'd never ask us to make different arrangements. They're thrilled to be going on vacation with us and they'd never expect us to pony up enough extra money to get them their own beds. And I honestly never even thought about asking for a cot.