What would seem 'fair' to me in any situation really depends on so many things. In our family, when we (DSis & I) started to be the ones making arrangements for family, rather than our DP's doing it, I always insisted we split the costs 50:50.
We started with a DP or DH each, it meant we paid equally for our DParents.
As her family grew with DC and DGC, I was still happy to pay 50:50, after all she carried all the costs of raising the later generations, without whom family holidays would be a much less rich experience.
Finding a way to make things easy on those who are carrying higher costs, without wounding their dignity is imo an important part of the wider family life
That is I suppose a focus on the benefit of the holiday, is all spending time together in relaxed surroundings.
The clue really is to find something that really does work for everyone. In the OP's situation, I'd tend to suggest that there might be an entirely different solution which might not have these tensions. Tension this early in the process does not bode well.
I'm sorry you are having this experience @Sallyrush In your shoes, I'd be tempted to skip the holiday this time. Hope you find a way forward that works for you all. It's horrible when a family falls into an ungenerous dynamic. It's very difficult to recover the relationships.