@Calcgirl I think you should have a talk with your son. Ask him why you are not included. Maybe there is a problem that can be corrected. But, it may have to do with the MIL paying or that the MIL is very territorial. When so many people are involved, there could be a number of reasons.
Also, keeping yourself busy with your interests, joining clubs, entertaining friends is important. We cannot control if our children include us so we need other interests to focus on.
I hope it helps to know that so many of us experience your situation, if not all the time, at various times. I've decided I can't control the situation, but I can control not giving it my energy. If my mind starts to wander down that road, I just tell myself to STOP and find an activity. Many times it helps just to come to these boards and realize there are others going through similar situations. Wishing you peace.