I think every family operates differently, and the OP is best suited towards understanding her sister's feelings regarding this stay.
As a little sister, I know, for a fact, I would have been fine with this arrangement, had my older sister sprung for my accommodations at Pop while she stayed with her family at the Poly. Why? Because I would have been at Walt Disney World. And I would have understood that money doesn't grow on trees.
I understand others may see it as a slight, but don't assume the OP's sister will. She could enjoy having time with her sister's family, while having an adventure at Pop all her own.