I'm about to become a stepmother. And no, I was not involved in the divorce. But it is possible that I will be involved in the biterness, anyway.
What do-I think about all this? Well, I dream of having a great relationship with my step children. They are kids, what happened in their parents' lives is not their fault, they have certainly more or less suffered from the divorce, and they have the right to their opinion (although I guess that their opinion is influenced by their parents' opinions while they are kids, so if their mother is not happy with our marriage, maybe the kids will follow). So they may give me a chance, or not. If they do, then I would like to be at all the big events of their lives nexto to their dad. But if this is not possible, or if there is real tension between me and their mother, I think I will skip the events and find a way to congratulate them when their mom will not be around.
I think this is fair for the kids, who deserve to have both their parents around every major event of their lives. This also a fair and not that big price for their parents to pay for claiming their independence from a marriage that did not work. As for me, well, I am not their mother. i could be a very good friend, and a very good friend ought to understand that she should keep out of the way to avoid the tension. Not to forget that I am not interested in the least in a catfight...
Were-I Prudie, I would advise the LW to try to have a talk with her father and explain to him that she would be glad to have a separate celebration with him and her stepmother, but that for the ceremony it would be better if she skipped it. I think it is more reasonable for the new wife to skip it (after all, she was involved in this story and has her part of responsibility) than for the mother, however wrongly distressed, not to attend.
However I agree that the daughter should not take blackmails. But this for me is another story, independent from the fact that adults should keep balance when the interest of a kid is in play.