KatyK I hope you are not offended by this, BUT your daughter and her family have come to you every Boxing Day and they have spent Christmas Day in their own home. They have not spent either Christmas or Boxing Day at SinL parents.
While that was happening, how did the other grandparents feel? You are upset because you will see them one of the days this year, but not the other. Did you wonder how upset the others have been previous years at not seeing them at all over the festivities?
OK, so DD has moaned about them in the past, but they have as much right as you do to see their grandchildren. We have heard so often on this forum how grandparents never ever see their sons family. If your daughter does not take her inlaws up on this Christmas Day offer, she will be contributing to depriving another pair of grandparents of some of their grandchildren's company.