It’s the condition, it’s insidious and it is like she’s being “possessed” by it.
I say this gently and appreciate you’ve been following advice, but from your posts I seem to see a pattern that the AN is winning this war in so far there is little incentive to get well.
Not using terms like “stabilising weight” as she prefers “healthy” - a term that’s much vaguer and down to her interpretation of what healthy means for example.
Demanding expensive food that’s difficult to get - to make it less accessible?
Not liking where she lives, so talking about moving?
Refusing to admit she’s lost weight and how’s she’s doing it - especially the timing given the holiday.
Again at the last minute starting to dictate how the holiday will unfold - what she will/will not do or tolerate.
It’s not “her” it’s the disease but I can’t help feeling that the control is with her. As such she’s seeing AN as a positive contribution to her life in so far it’s enabling her to extend that control across every aspect of her life - and frankly the rest of the family.
I’m so sorry your dealing with this. However maybe this is a turning point. An opportunity for her to realise that her condition isn’t “beneficial” - it doesn’t give her positive outcomes. It won’t be an overnight revelation, and likely to get worse before it gets better but I’m not sure filling her life to distract her from AN is a strategy that would ever work - essentially because it’s rewarding the condition.
My guess is she will lose more weight whatever you do. Once you are on holiday she has learned boundaries are not going to be upheld so she can wield control as she sees fit over every aspect of the holiday - this in addition to the fact she maybe physically unable to cope with long days in the park.
If you leave her, again she’ll lose weight. I don’t see a good outcome either way.
However there is a good outcome for your other DD. Which is to go on holiday.
If you feel unable not to take her - and I appreciate this is a very tough call to make, then I think you need to sit down with her and lay out some very strict rules for the next week and the holiday - and 100% stick to them.
I’m not convinced that will work though - it’s back tracking and I think you’ll get lip service in agreeing and once on holiday you’ll see the manipulation and need to assert control come to the fore.
I’d be tempted to speak to her therapist again and/or other specialist support organisations to talk this through.