I disagree with Prudence.
I would be upset. I don't know about the wedding, but I would certainly skip the engagement party.
I also wonder what else there is to the backstory. I admit that my knee-jerk reaction is to assume that there is a long history of the brother one-upping the husband and always having to be the center of attention. Because, really... 365 days in the year and you just happened to pick the one that is your brother's wedding date? And to plan a major all-about-you party on your brother's first anniversary? Really? It sets my hinky-meter off.
As someone said upstream: If you want to have a wedding on the weekend in the spring, you don't have 365 days. You have, what, 12 weeks x 2 weekend days = 24.
In 2013, May 11 is on a Saturday. In 2014, May 11 is a Sunday. (Last year, May 11 was a Friday.)
I was going to say I think it's weird to have an engagement party that far in the future; I think they should be planned for soon--but if their culture has big, splashy parties, then if they announced this in the most recent couple of weeks, it's less than 3 months ahead.
The thing that might hurt *my* feelings is that they clearly didn't care whether I was at the engagement party. And I would already have a pretty strong commitment to spend that evening/day w/ my spouse since it's *MY* first anniversary. And the reason they don't know that is *not* that they didn't remember my wedding date (I wouldn't expect them to)
The reason they don't know is that they didn't check with me to see if I could make it. And that would sort of hurt my feelings.
I know you can celebrate an anniversary on a different date, but I really enjoyed celebrating mine ON the date, on the REAL date. With just us. Later anniversaries, eh, who cares.
So I'd be a little hurt, but I don't think I'd truly consider it completely wrong, no qualifiers.