I don't know, I might ask R if there's any way she could change her mind. This day will never come again. Why on earth would she even want to go to the wedding, when she doesn't like M? Why is it ok for her to go to parties that you're not invited to, but not the reverse, when it would be so emotionally significant to you? M and D are within their rights to not want anyone who doesn't whole-heartedly love and support them at this special event, but why does it matter so much to R? Some things are perfectly ok to forgo for your partner, but a one-time-only event that matters to you but that has no reason to matter to her doesn't seem like one of them to me.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.