My brother and I each separately told my mother to spend her money on her own needs, not to worry about us. (Amazing, as he was always playing poor.)
But on the subject of the parents thinking their ideas are fair, and the kids not feeling the same, she wanted to give me all her funiture and belongings and all the cash or similar assets to him. After all, she said, her things were more valuable. Not. And though she treasured that old furniture or clothes, I did not.
I kept a few things that mattered, but gave him free reign to choose. As I said, we split the money.
But OP asked about the present. Would you pay for one’s hotel room and not the other’s, because one earns less? Obviously, first it matters what the parent(s) can afford. Assuming they can easily pay for the rooms, why not pay for all? In this case, the trip is presumably pleasure to all. Is it “unfair” to the poorer sibling that the other also gets a free room? I think not.
But I do think the poorer one needs to adjust her expectations to the fact that her choices (or circumstances) don’t allow the same discretionary or free will spending as the one who can afford more. If one sis gets that expensive handbag, the lower earning one may have to wait until her own finances allow. Nor do I think the better paid sibling “owes” it to buy the other a gift equal to what she bought herself.