Things are better now that we have children but were sincerely awful prekids. I try to be the best wife I can be to DH and I wish that his family had more of his wonderfulness so that is what I work toward by biting my tongue, turning my cheek, and ignoring their bad behaviors.
If it was just his parents that would probably unpleasant. It is also one of my DH's brother and his family. His brother is okay. His wife doesn't speak unless directly spoken to and does not really understand reciprocal conversation (I have fully explored the "very shy" vs "just unpleasant" difference over the past 6 years or so and have a firm conclusion). They have a kid 1 month older then my oldest and a brand new baby. So, 4 under 3 with 5 adults.
All of the planning has been left to me, despite my, "Here are five great websites for planning, here are five restaurant review sites, here are forums and more!" they have not done a thing. I have run with it, reserving table service meals for one meal a day and deciding where we would stay.
My plan was to go to the waterparks--we did Disney last September and it was sweltering. I told DH we could go this September if he promised he would never make me go then again--we have the super duper annual passes and I want to try something new. No problem for me to take mine to the waterparks (even solo) but I am positive it will be an issue to everyone else. I am sure they will refuse to split up from me. I am also certain they will look to me to decide everything.
No big deal (oh, except for the HEAT) it just isn't what I prefer. No one is honest but everyone feels free to backbite and complain. All along my emails have said, "Please yourself. Figure out what you/your family would enjoy and do that!"
It all comes down to me saying, "We are going to waterparks. I know you haven't been here before but I'm sure you can do MK just fine on your own. If you'd rather come with me that is fine but no complaining!" That will be hard but personal growth is not something I shy from.
I am trying to be hopeful that my pretrip dread will be the worst part and that I will be pleasantly surprised by how not terrible/awful/will Southwest let me come home early the trip actually is.
Thank you for the support and GREAT suggestions. This trip was my hope (and idea so I only have myself to blame!) to further relationships between my kids and their grandparents and cousins. I knew even with DH along that it would not be a vacation per se. But I would walk through fire for my kids (and my DH!) and if I look at it as an opportunity to grow myself and help my kiddos have better relationships with DH's family I am certain with prayer, luck, and pixie dust I can accomplish that.

I just may need a vacation from my vacation when I get back!!!
SotS