Many moons ago my DH and I celebrated our 1st anniversary by staying a the same place we spent the first night of our honeymoon. It was really special for us and important that is was on the date. Subsequent anniversaries haven't been as important. Heck, this year we had a 7 month old and a 3 year old and had take-out Thai after they went to bed.
But, the first? Yeah, I wouldn't want to spend it at someone else's engagement party. So I can understand the write'rs point with that.
As to expecting someone to remember the date? I am terrible with dates. I sometimes get our anniversary mixed up with the previous weekend that we were also considereing. heh. So I don't know that I would necessarily expect the brother to remember the exact date. However, I would think that he would remember the month and that it might occur to him to look up when his brother's wedding was. But I can't necessarily fault him for not doing that.
So I think the LW's husband would be fine telling his brother "Hey, sorry about your engagement party but we already have plans as it is our anniversary." If there isn't some sort of rivalry between the brothers, I would hope that would be understood.
Although the cynical side of me wonders how the brother would feel if the situation was reversed. The date is clearly important to him. Would he be willing to spend his first and second anniversaries celebrating someone else's marriage? Who knows...