I think it's silly to be upset by it, and I think it's petty and unreasonable to skip out on your own sibling's wedding over something like this. It's not an etiquette violation, but I'd think it was a pretty selfish way to behave, personally.
Perhaps that date has even more significance for the brother and his fiancee than for the LW and her DH. Perhaps they had always been planning on getting married on that date, if they did decide to get married, and when the LW and her DH took the exact same date THEY were surprised. Or perhaps the venue of their dreams was only available on May 11 during the period they wanted to marry, so they decided to make a thing of it. (If you want to be married on a weekend in the spring, there's only so many of those.)
Sometimes important things happen in the lives of those around us on dates like our anniversaries, our birthdays, etc. If we care about those people, we'll be willing to celebrate our birthday/anniversary the day before, or heck even the weekend before, in order to participate in the special event. If we don't care about the people in question that much, then it doesn't really matter.
I tend to be of the belief that current events trump anniversaries of past events. If everyone's anniversary/date of death/birthday/etc were off limits for everyone in the social circle, there would quickly be no dates left to do new things on.