I'm with the sisters on this one. You don't plan a get-together during another when people are visiting from out of town, have a limited time to visit with each person, and are relying on you, as the locals, to plan their gatherings.
It's just not nice, sorry.
My background is that my paternal and maternal families were of different faiths, and the one side didn't respect or care about the other's traditions. So every other year or so, they'd plan a Hanukkah party ... on Christmas day. When it's an eight-day holiday, and sometimes they'd even do it if Hanukkah had been over for a couple of days, or even if Hanukkah spanned a couple of more convenient weekend days. They know what Christmas is, they know my family celebrates it, they just thought it was funny. We went the first year they did it, thinking, "We can't miss the annual Hanukkah party!" and ... never again. Christmas wasn't "more important" than Hanukkah, but it was only one day, and it was disrespectful of them to be inflexible and unaccommodating of our side of the family.
I understand that your husband doesn't like their parties, or something. And I understand that they're not willing to bend over backward (or even do anything) to accommodate you, but expecting them to do what you want just because you planned it isn't fair to them.