Im so sorry this has happened to you OP. I hope you are OK.
I think the Aunt deliberately left her "announcement" to the very last minute to be sure of winning her way, because your Partner and the Bride were probably in a panic that it would overshadow the wedding and cause a huge kick up on the day. And it has left them virtually no time to discuss it or even think about it properly. If they'd had more time, they could have got some of their other relatives to tell the Aunt her behaviour was unacceptable. And then the Bride would have know she had the support of her other relatives.
I had worries about several of my relatives attending my wedding and was basically praying that they would behave themselves. (Predictably some them did not behave well, but another relative dealt with it and I didn't know until afterwards.) I can imagine that this has really upset the bride and your partner as well as you.
However, I do feel that your Partner should have contacted the Aunt and stood up for you, and that it was blackmail and seen if he could talk her round. I would feel let down if he hadn't tried, but then it does depend on the personalities involved. A disagreement a month before hand wouldn't have so much impact and could be got over but a few days is a different matter.
Maybe she wasn't the type who could be talked round, and perhaps he already knew it was fruitless, we don't know what kind of relationship he had with her when he was married. The Aunt might have been longing for a confrontation and a big walk out to do a drama grief display and make the wedding all about her. She probably knew that if she didnt come or walked out that the Bride would have to deal with a whole host of relatives wanting to know why. And it would be grim to be there yourself whilst that was going on. And perhaps this is what the Partner and Bride were anxious to avoid. I'm guessing it would be very hard to deal with someone who behaves like that and her last minute blackmail points to her being that sort of unreasonable person. And it's not as if he could have said to the Bride, well in that case I'm not coming without my partner as that would be just as bad a blackmail as the Aunts.
I think I would want to know a lot more about how and why he dealt with this in the way that he did. And also to find out if he understands/cares about how much this has upset you, or whether he thinks you should just get over it. And if he offers to pay for your wasted outfit (I imagine it would be hard to wear it now)
I hope you manage to have a calm, relaxing day.