Ah you’re pivoting now that you’ve seen your argument that DS was potentially losing out is bogus. Good-oh.
DuchessDarty - Sorry, I actually hadn't seen your reply when I posted that!
I do get your argument that he won't have to share or get the box room, but the not sharing is just by virtue of all the other siblings including the baby being the opposite sex, which isn't his fault or choice.
I guess he could be considered for the box room, but I don't really see the logic in that if it's just so that SD can have a bigger room to herself instead. They are the same age and he lives in the house all the time. I genuinely can't see the sense in him having a much smaller room just for another bigger room to sit empty half the time.
But putting everything aside, I genuinely think the parents need to make a decision based on what's practical, not on what suits one child the best.
That to me is either SD gets the box room to herself, or she shares the bigger room with DD6. Putting myself in her shoes, I'd much rather have the box room to myself, particularly thinking over the next few years when she is 13/14 sharing a room with an 8 year old who's going to bed much earlier.
I think she has just seen the size of the room when it's empty and been unhappy, rather than being helped to consider how much better it is for her to have her own room rather than sharing.