I so know how you feel, although I am kind of in the opposite boat. My in-laws are here and can see us any time they want (they don't), and any holidays they seem to think need to be up at their house. My family is in California, and I see them maybe once a year.
We just TOLD my in-laws that we have Thanksgiving in CA. (Thanksgiving is easier because we have the long weekend, where Christmas always makes it harder with work.) They might be mad, but it is just a date, if they really feel that strongly about a holiday, celebrate it at a different time. We also complicated things by now saying that Christmas is at our house, and they are welcome to come. We don't want our son to have to go through what I did as a kid - here are all your presents, leave them alone, we have to go spend the day at a place that isn't babyproofed, doesn't have a place for you to nap, and all they do is watch TV.
My father, also in CA, expects us to go to their "other" house. He doesn't talk to the rest of my family, and won't participate in a group anything, and gets all upset that we want to see the rest of my family if we are going through the expense and effort of going down there. (He has been up here twice since I moved - my wedding, and my son's baptism.) I was able to talk him into doing the group thing one time, and he seemed to understand the logic of choosing 10 relatives at a dinner instead of 2. There just isn't enough time to make those individual visits, especially when there is travel involved. He has since lost that logic and isn't currently speaking to me, so he won't see us, or his grandson come Thanksgiving. It makes me sad, but that is his decision.
In the end, I have decided that it is their decision how they react. I have to do what is right for my family (my immediate family), so if this year, it is to be with your father-in-law, which is totally understandable with a recent death, your mother will just have to get over it. And if not, then oh well, there isn't much you can do.
Sorry, that this probably isn't much help, but just wanted you to know that you are NOT alone.