wait, so you'd spend most of Christmas day and pretty much all of boxing day with PIL and they still think it's unfair that you'll want to spend any time with your Parents? bloody cheeky if you ask me! It seems like she's not thinking about your family being part of yours and your DP's family at all. She might just see him as her son and you are a new addition to the family, but failing to comprehend that her son is also son-in-law to your parents, and as much part of your family as you are part of his (not sure if that makes sence!)
If they haven't been around at Christmas since you've been together (or had to plan through a wedding or other extended family events like a Christening), that her son has a new family to consider as well might not have occurred to her. Have you asked your DP, did they go between the two sets of grandparents as a child? That might be telling. (If they only spent it at home or went to one side, the idea of balancing families might not have occurred to them either).
Also, if SIL's PILs are so far away that she has to do the whole of Christmas with one side or the other, then again, the idea of fitting everyone in not have occurred, with SIL it could be she does "all or nothing" so PIL think if you are going to them for Christmas, it's the whole of the christmas period, not just the day...
anyway, just because MIL hadn't thought about this, doesn't mean she gets to make it your problem to give herself a dream Christmas. Tell her when you can go from your parents to hers, if she doesn't like it, you'll skip it and go to your parents, or you could offer to host and have everyone at yours... (of course, that way maddness lies...)