Am sure there are loads of suggestions here, but wonder whether there is a PDA element too?
The starting point is to have no plans - no big trips or sight seeing planned, just the possibility of doing so.
Is it worth sitting down with her and asking her to plan your day with you and structure things with open questions so that there is no ‘demand’ element?
eg “today was very stressful for you, I can see that, would you like to plan what we do tomorrow?’
Dad would really love to take us for pizza in local town - would you like a pizza? No? What would you like instead?
Mummy and daddy would like to go into town for a little while to look at the pretty castle/roman mozaic - shall we do that tomorrow or today?
can we buy you an instant camera so you can take some pictures while we are out and then we can take them to the shop to be printed when we get home? You could be our photographer?
Would you like a quiet day at the hotel/villa tomorrow or shall we take a walk on the beach after breakfast?
Would you like to play in the pool before lunch or afterwards tomorrow?
Oh you don’t want to play in the pool - do you want to bring a colouring book/the ipad and sit with mummy under the parasol while daddy plays in the pool with sibling, instead?
I know your toys are very special, which toys shall we keep just for you today? Is there a toy that you think your sibling might like to play with that you could let her have today?
Ie, you ‘empower’ her to make decisions, to feel part of the process. It’s not gentle parenting in anyway, but a way to remove conflict and to get her to actively engage in what is going on, to ‘buy in’… thereby reducing the stress for you both?