Given the 3rd hand way you received the info, could there be a misunderstanding here? Is it personal about your mum, and actually asking that your mum doesn't come, or is it turning it down as they don't fancy Christmas day with people they barely know so have decided to give it a miss?
I can easily imagine accepting an invitation under the impression you were seeing your DHs side of the family this year (perhaps because bil hadn't fully explained to sil the plan?) And then finding out that actually it was a larger event with people I barely knew and that changing the dynamic, so wishing we hadn't accepted. Is she quite an introvert maybe? Plenty of people on here like to have Christmas just their little immediate family.
However, that wouldn't excuse expecting you to uninvite your mum, but did she actually say that or did she expect bil just to send their apologies because they've decided to stay home this year, and he's out his foot in it mentioning the reason and explaining it badly?
As for only mentioning your mum not dad, I can easily see that bring almost a shorthand for both parents that's been lost in translation of the Chinese whispers along the way?
Or of course she could be totally unreasonable!
Either way, don't worry if they don't come, it's not a big absence. We get on well with our siblings but hardly ever see them on Christmas day since we were married as generally if we are seeing one set of parents our siblings are seeing the in laws so no parents are left alone. So it's not a huge absence when you have 3 other guests.