My dd2 (4) has a tiny bedroom with no space for anything except basic furniture. My dd1 (6) has a much larger room, with lots of their joint toys stored in it, so naturally dd2 likes to spend lots of time playing in dd1's room. Usually this isn't a problem, but every now and then dd1 decides that she wants her own space and wants to shut dd2 out of her room.
This afternoon dd2's friend is coming to play and dd1 will be out. I suspect that dd2 and her friend will want to play in dd1's room, but I'm not sure if it's completely fair on dd1 to have people taking over her space when she's not there. I suspect that as dd1 gets older she will increasingly want her own space.
So, if you're in a similar situation how do you deal with it? Do you let the boxroom child have full access to their sibling's larger room? Do you have rules limiting their access? Or do you let the siblings negotiate it between themselves?