DH has just turned 40. We usually have a celebration of big birthdays in the summer because nobody can be arsed at this time of year. Rather than a party, which he definitely doesn't want, I've arranged a surprise weekend for him and his 3 brothers. It's a specific thing that's only on for that weekend, and I checked with them
all that they were free for it last spring. All confirmed they were and I bought the tickets in April 2015 (ie 16 months in advance).
Now, of course, I knew something might come up at the last minute that might prevent one of them from making it. It's come up in conversation today that one of them has been invited to a wedding abroad that's on the same weekend. He's trying to find a way to do both, but in reality it will mean he makes 1 day of the thing rather than 3. Surely knowing that he would be away with his brothers that weekend he should turn down the wedding invite. 
AIBU to be annoyed? WIBU to point out that he agreed to this first, and that his brother should take priority here?