Wow, Prudie really raked her over the coals. I thought the comment about no one caring about LW's anniversary until Congress makes it an official holiday was especially mean-spirited.
Even if it was a thoughtless oversight on the part of the newly engaged couple, with no malicious intent, I still don't think there's a problem with LW and her DH skipping the engagement party.
Different things are important to different people. I like celebrating my anniversary. I don't usually remember my birthday, but I make sure that we do something for the kids. For my anniversary though, I'd like to get out of town, or get a sitter and a hotel room. Something special for just me and DH. Things get crazy, and we don't get whole days to spend just the two of us very often. Um, actually almost never. LW may not have the child care issues that DH and I do, but they may very well want to go out of town, or spend the day where they had their honeymoon. A friend of mine has spent 3 of her 6 anniversary weekends at the cabins they honeymooned at, 3 hours away. You can't do that and make it to a wedding the same weekend. And in one fell swoop, LW sees her first two anniversaries 'dibsed' for BIL's wedding. If it were me, I would worry about it being a sign of exactly where I stood in my new family