I would love to know if there's any back-story here! Is May 11th a special day for the entire family, is the brother clueless, or is something else going on?
In my family, any of us would be honored to share an anniversary date... I nearly got married on my brother's (6th) wedding anniversary, but friends of ours chose that date before we did.
The difference is that I knew it was his anniversary, and spoke with him and my SIL about it first while we were checking dates with our immediate families. They were THRILLED at the possibility.
But now let me tell you how my MIL ruined my first anniversary...
She made a point of asking about our plans weeks ahead of time -- dinner at the romantic restaurant where we had our first date. A few days before the date, she announced that she was holding a 50th anniversary party on that same day for DH's grandparents... except their anniversary wasn't for 2 months.
Still, since that was a lunch event and we were planning to go to dinner, we thought we could do both. We gathered at noon, and waited for food to be served. And waited, and waited. We finally ate at 4 PM. By then we were starving, but knew that if we ate we'd ruin our dinner out.
MIL forced food upon us (it's really hard to not eat around her, particularly when starving!), DH caved, and that was that.
It was absolutely deliberate -- the rest of the family was equally confused about the rush to hold a party for the grandparents -- and ruined what could have been a special day for us.
My MIL is the kind of person who delights in creating this type of situation to exert control over her children, and it took years of spine-strengthening before we became immune!