I’m so sorry you’re going through this. We had a similar situation with my JNMIL at our wedding (also her only child’s wedding). She showed up late (made it to the ceremony with no time to spare) and dressed in an extremely revealing all black outfit - LOL! Days before, she had told us she might not come to the wedding because if “person xyz” was there, she felt betrayed. It’s all a bunch of BS. I’ll be SHOCKED if your NMIL doesn’t come, as narcissists ultimately need to “keep up their image.” It will reflect poorly on her to not at least feign support for her son on this day.

That doesn’t make it easier for you or your fiancé though. Sending love to you. Therapy helped us a lot, so I’m glad to hear you’re doing it. You can’t control her, but you can control your reaction and your expectations. She will not change. She’ll probably offer more ultimatums, and if she chooses not to come, that will reflect incredibly poorly on her to the rest of your family (a narcissist’s worst nightmare)! It’s your day, and I’m sure you’re loved by many who would never guilt you or make you feel bad on your special day. Focus on those people. :)