Yes, this is baffling me as well.
It's not like the eldest is grudgingly being given a grubby coal scuttle in the back yard to bunk down in; there is a whole bedroom available for her, that she can gladly use whenever she wants to stay over - the same bedroom that has been considered perfectly adequate as her younger sister's full-time bedroom for years.
On countless stepchildren threads, posters always say that a child who lives with their DM in their own big bedroom most of the time shouldn't expect to have the biggest room for one night a week with their DF, relegating his DC with his new wife - who live there permanently - to the smaller rooms.
Obviously, a big factor there is that the (dependent) children never asked for their parents to split up and remarry, so that they would live between two homes, or for any new half-siblings to be born; but in this case, the eldest (adult) child has chosen to move out!
At what point does she accept that it is no longer her permanent room? Will she still expect it to be set up and reserved as her room when she's 40? Will she expect a greater inheritance, when the time comes, because a larger part of the family house being sold is 'hers'?!
I realise that she's finding the transition a challenge, but she needs to understand that her sisters - who are younger than her, too - can't be expected to live less nice actual lives in order to accommodate her own idealised two-home life.