I'd be tempted to take my kids and go have a holiday on our own on the other side of the continent! :LOL
What an extended family, and what a sense of obligation you've ended up with to 'share' the kids around to all - that sounds tough.
I guess if your ex only sees his kids once a year, this is 'it' for the year for them, so if I were you (and barring any big issues, like abuse or whatever, that you didn't mention) I'd probably try to facilitate that, within reason. He didn't give you dates yet, though? I think I'd just keep in mind that he'll be 'around' when making plans. Decide who you want to see and when, leave a few gaps here and there, and then when he gives you dates, you can say "Oh, well on the 23rd we'll be at my mom's (or wherever) in the evening, but the boys can see you in the morning if you like," or "We'll be at your folks on Christmas Day, so you can come by there, or if you don't want to do that, the 27th works for us." If you want to spend time/have the kids spend time with your ex-in laws, I definitely wouldn't scrap that just because he's coming.