R.,
Do what you need to stay healthy, and limit the family drama, but you remember: YOU are the one in control of your emotions, you are the one who can avoid feeling rejection if your family is acting up. Control your emotions and you have a great hurdle overcome - no matter what they do. I find that I have to get over the feeling that 'they don't do what everyone else's family does or what they are supposed to do'. TV really does instill a sense of something missing when life is not like a sitcom. (I think it is short for SITting for COMmercials).
I guess I would have opened the invite just to see what it was for. An invite to a shower you may or would not want to attend is also something you want to think about: You will have a niece or nephew born into this craziness, and in years to come, may want to have a relationship with you (out of self-preservation).
I think it would be important to keep the door open to possibilities for that child, even if the mother and grandmother are removed from your life. Family IS family, and I think you have the right key: love from a distance is healthier for you.
My heart aches when I think that you are going through what my Mom went through. She learned to cook at 8 and went on being Mom without the title until she was in college and for a little while when married. The rest of the family plays games too. Sad, but just what it is - you can't change others, only yourself, and I don't think anyone has the patience to try to break down that barrier in a healthy way. It has to come from both sides.
Be careful they don't try to get your oldest in on things too. My grandmother snared my younger brother. He now talks to my Mother and I like we are stupid.
I pray your situation changes, but for now, be your own best friend and family member and protect your children from the craziness. You're doing a great job it sounds like. Keep up the good work. Health is important ON ALL LEVELS.
Good luck, let me know how it works out if you want,
M.