Yes, she has needs and challenging circumstances; but I still don't see why her younger sister has to give way to make allowances for them indefinitely.
The room was never going to be hers forever, whether younger siblings moving up to it, mum and dad moving/downsizing or whatever.
Those challenges need to be approached at some point, and I would have thought that her moving out would be the logical time to raise it. It's a positive thing that she's grown up and moved out to a place of her own, although she's always welcome to visit and stay back 'at home', of course.
If they don't do it now and let it linger, that will cement in her mind that it is her room forever, so how is she going to react when she's been living in her own home for 6 months, a year, two years, five years?
Meanwhile, her youngest sister has been waiting for 16 years for a decent sized room. She may not have autism or ED, but she will still have a natural sense of fairness, of mattering and of being considered equally important to her sisters. It's not her fault that any move will be a 'bettering' - that's only because she's already had the worst for all of this time.