My second wedding anniversary was two days ago. We celebrated by going to the dentist on Monday, going to work on Tuesday and then eating dinner out at a chain restaurant, and we'll have our "real" celebration on Saturday.
But for our first anniversary, we went away for the weekend. It was really, really important to us to do that, because we both thought that it would set a precedent for how we celebrated all future anniversaries. Even if we couldn't take a trip together for every one of them, we felt that it was important to set the bar high (knowing that there would be years, like this one, where it just wouldn't be realistic to do more than go out to dinner).
For me, my anniversary is not just a significant date. It's an annual reminder to me that I need to spend some time focusing on just my husband, to the exclusion of everyone else. It's important for the health of our relationship to do that every once in awhile, and it can be easy to forget to in the normal course of day-to-day life. But because that's the viewpoint I hold, I am not able to celebrate my anniversary unless my husband and I are alone. I wouldn't really be celebrating it if I ended up attending someone else's wedding--I'd have to do my actual celebrating on another occasion (and lunch the day before an evening event probably wouldn't count, because our focus would be elsewhere). From this point forward, under most circumstances, I'd be fine doing that, but I wouldn't have been on my first anniversary.
Given all of that, I think it's unfair to chastise the LW for not attending the engagement party, and I agree with what Toots said much earlier in the thread that I'd be a little hurt that nobody checked with me before setting the date to make sure I could actually be there. I think that the LW might be feeling hurt about the wedding in part because it and the engagement party are both on her anniversary. It might have been less of a thing for her if the engagement party had been planned for a different date.
Personally, I can't really fathom not even remembering the month a sibling got married in less than a year after the event. I'd have to really think to remember my brother's wedding anniversary, and I don't always remember exactly how long he's been married, but I know what month it was.
I also think it can be a lot easier to share a significant date if the person planning a new event both remembers and acknowledges that something else happened on that date, too. Even if the date is pretty much set, I think the approach makes a difference. Having a sibling announce a wedding date as a done deal without having talked about it with you first can be very hurtful. If the date itself is also significant to you (and their wedding changes how you would mark it), that would only add to the sting.