The problem is - and I hesitate to use the word problem - comparative situations aren't really working here. A toddler sibling climbing into bed with an older sibling or with anyone for that matter is different because toddlers don't generally have a clue about social norms. My toddler is still in a nappy, would happily strip off naked at the pool or the beach if I let her and will scream hysterically if someone does something heinous like button up her cardigan or try to hold her hand.
It's also not comparable to a holiday situation, although I have just remembered when we went on holiday when I was 12 and my brother was 14, we all had one room with two double beds and I shared with my mum and dad with brother, and believe me our parents were generally very off kilter with what was age appropriate! But then the excitement of being on holiday and the fact normal 'rules' don't apply makes it really different.
When I initially replied, I wasn't thinking along the lines of sexually inappropriate behaviour. It's more about boundary setting. This one is particularly important at this age because certainly the older one is on the cusp of puberty and no, that doesn't make him for a moment a sexual predator, but it does make him old enough to understand (gently) some things are not appropriate. Sharing a bed with someone when circumstances mean it's necessary is fine. Requesting it every night when you have a bed each suggests something is going on. I'm not thinking along the lines of sexual exploration (although it's possible) but is someone frightened? I saw a film when I was 11 that terrified me and hated sleeping alone for a bit. Is it anxiety about school or whatever?
The thing is, this isn't sharing a room, and it's also deliberate - it's not sitting on the bed watching a film and falling asleep. That gently suggests to me that the older one in particular isn't really understanding boundaries too well and he does need to.
I think for a lot of posters, they see this sort of thing as a sign of innocence, as if the tainted standards of the world haven't touched their fresh faced little poppet yet, and I don't think that's necessarily a good attitude. Becoming aware of your own personal space and other peoples, of your own sexuality and seeing that as a positive - becoming a young man or a young woman - and one who treats others with respect due them.
Ignoring the fact that your children are growing up all the time and trying to return to sugary days of toddlerhood doesn't work. I know some people like to breeze it away with 'whaaaat, they are children' but there are ages and stages in childhood just as there are in adulthood: at nineteen I was happy to do things I wouldn't appreciate now like sharing a house, or going to bed at 2 and being up at 7! It's not a big drama, not an end to innocence or the death of childhood, just yet another new journey. :)