I think that if a guest, by only staying for part of the party, substantially changes the dynamic of the party, then it's rude.
In this case, I think it depends on how many relatives were only dropping in for an hour, and how central their attendance was to the party as a whole. It kind of sounds like despedina ended up having to change around the order and timing of events so that her MIL could be there for the meal and present opening. If she did that on her own initiative, I'd say just don't bother making changes to the schedule the next time she invites MIL to something (and if the in-laws miss out, that's on them).
If she changed things around because the relatives who only dropped in insisted on it because they were only going to be there for a short time, I think despedina's response (not doing these types of parties anymore) is entirely reasonable. She could just refuse to change the schedule for people who don't stay for the whole party, but that would be much more stressful (and likely confrontational) than just letting the kids do something else to celebrate their birthdays.
In the end, despedina gave ample notice of her party to allow her guests to plan their schedules. Behaving the way her in-laws did (accepting her invitation well in advance, then accepting the anniversary party invitation 3 weeks before both parties and choosing to spend the bulk of their time at the anniversary party instead of at the birthday party) absolutely does communicate that the birthday party is less important. And maybe it's justifiably less important, but making it so painfully obvious to the hosts and perhaps the guest of honor is extremely hurtful and a little rude.
Plus, an anniversary celebration might be a bigger deal to a lot of people than a teenager's birthday party, but not if the anniversary party isn't even planned to the point of inviting people until 3 weeks in advance. If it matters so much to the people celebrating the anniversary, they ought to make their plans known quite a bit sooner. As someone else said upthread, it's not like they don't know the date. I would personally assume that my presence was much more sought after and valued at a birthday party I was invited to 3 months ago than at an anniversary party I was invited to 3 weeks ago. Even if I thought I needed to attend both, I'd do the drop-in-attendance at the anniversary party, not at the party I'd made plans to attend weeks before.