Namechange for this because I find it quite triggering.
It's fascinating that this thread has run to 7 pages since it opened yesterday morning. It calls out to just one issue: the OW and "get over it"/"I can't get over it."
Is it too hard to understand that when a woman has children and her partner leaves her for someone else, whatever sorrow, betrayal, rage, or increasing sense of freedom she may feel, at some level she often intuits that she has been usurped by another, better, more attractive and exciting (often younger) version of womanhood.
She loses her husband/partner to this usurper.
She may try to be rational/sane/her better and bigger self and encourage her children to have a good relationship with this OW as their SM, but at some level she often fears that she will be usurped by another, better, more attractive and exciting (often younger) version of motherhood.
Unto the last generation - grandchildren, great grandchildren. Shoved into a smaller and smaller corner, because her hopes and aspirations were dashed when she was young and she is increasingly diminished by her failure to match up to the usurper (to whom none of this matters a damn, because apart from her relationship with her DH, the SM has no skin the game: it's just a popularity contest for her).
And, yes the first wife/the first mother may cut off her nose to spite her face and refuse to attend a grandchild's birthday party. She already knows her face doesn't fit.
And of course all the stepmothers/OW/people who have never had to consider how this sort of shit fucks with your head, all of them buy into the idea that the first wife/mother is a bit bleugh. That fits their agenda very well.
So, OP, think ont. A step-grandmother is not a grandmother. To confuse the competing rights of these two parties would be to undermine your own role as a mother. You have skin. And it's not a game.