I'm on your side, OP!
You had made your plans around your DIL's family and then they changed their plans to coincide with yours. I'd be irritated too.
I would rather be on my own as well. If your DIL and her family refuse to let you do his side/ her side holidays, I'd resign yourself to either doing holidays at a different time or taking yourself totally out of their holiday equation.
Your son is stuck with them as relatives now, but you aren't.
My parents and my inlaws don't get along very well. Everyone sucks it up for joint family occasions like my kid's birthdays etc. no problem. However, I would NEVER be selfish enough to expect them to spend holidays together to make it easier on me. We either skip spending the holidays together, alternate families, or celebrate on alternate days.
I'm guessing, since they specifically changed their plans to be there when you are there, that this won't be just a holiday issue. THEY want it to be a joint occasion and want to be there at the same time you are. Hopefully your son and DIL will be willing to travel to see you at times, because I have a feeling that everytime you go to their house DIL's parents will decide to show up too.
While it might not always happen on holidays, it is not unreasonable to want to be able to visit your child at least once a year without his inlaws present.