Hi Struthruth! I think I get what you're feeling. You've been treated badly by your xh's 2nd wife, and you feel your ac should stand up for you against her. When they cater to her wishes, it makes you feel disrespected. Have I got that right?
If it's any comfort, though, I think your ac tried to accommodate her for their dad's sake when he was living. Now, she has become part of their family and their children's lives, and so, Imo, they accommodate her for that reason. Whatever her faults - and they are glaring, imo - I'm guessing she loves the kids, and children can't have "too much" love. So they keep her in their lives and respect her wishes regarding you to keep peace with her.
As long as you see them and the gc, also, I agree with others that you're doing ok. Really, would you want to be around this rude woman more often? I wouldn't! In fact, maybe your ac think they are doing you a favor, as well, by keeping visits separate.
I may be wrong, but I suspect you have a vision of a "big happy family," with both you and sgm (stepgrandma) at events, etc. with your gc and their parents. And, apparently, that happens, sometimes, but sgm isn't friendly when it does. Imo, you need to let go of that vision and accept things as they are. Please just enjoy your time with your ac and families and never mind how things "could/should have been."