I have read I think, most of the posts on this thread, and it seems to me that there are two, maybe three, major factors here. On one hand, I cannot conceive leaving a close family member out of the festivities, under any circumstance. If there are now beds, I would get an inflatable mattress and everyone would be sleeping in my house, regardless of how tight it might be. The "no room at the inn" concept doesn't wash, after all, (and I am not religious, mind) was that not the reason that Jesus was born in a stable?
Two; the reason given for leaving the OP out, to a certain extent, seems to be based on the fact that "she doesnt enjoy Christmas as much"; She certainly isn't going to enjoy it more by leaving her to one side. Also, having lost a child at this time of year, would make the celebrations pale, methinks. Another reason not to leave her out.
In my family Christmas is a big thing, and my kids all flock home for Christmas. Until now, this has not been an issue, but if it were in the future, I would imagine that we would reach some sort of consensus. I know my eldest has staunchly told her boyfriend that she wishes to be here for Christmas day, and I know that she has arranged to spend a different festivity with his family (in this country there is also a big celebration on January 6th, the day of the Three Wise Men) and, having consulted the boyfriend, he tells me that for his family this day is more important. Were this not the case, I would have talked to his family to see how we could arrange for all to feel included, and would have opened my house to his tribe too.
I have often had friends, and even acquaintances sharing my Christmas table, if they were going to be spending Christmas alone. Some of the most special Christmas days in my life, have included people who were not in my immediate family.
To Mary, I hope you do have a lovely day, and that your walk with your son and grandson makes up, in part for your hurt. I think your son offering this may well be his peacemaking gift, as he is in between a rock and a hard place, poor lad.