I think your husband has to talk to them. There may be times in life when you do need to be flexible, and a routine really isn't everything. No one will be harmed if things get a little wobbly for a day or two. For example, a nap at 11am instead of 10am because of family obligations. But at the same time, you have to look out for yourselves first and if you're exhausted already, you need to get your LO to bed and go to bed yourselves to get through the night, then that's what you need to do. We often took a two-pronged approach: if there was something going on (wedding, family dinner, whatever) that would have just been impossible for us all to make, then my husband went and I stayed back with our daughter, made sure she napped or went to bed if bringing her along wasn't a realistic option. That way, one of us showed up and represented the family, but it didn't screw the whole night up by keeping her awake when she was really tired.
Otherwise, sometimes if it's truly not going to work, you just have to put your foot down and not show up to things. One year when our daughter was 10 months, we went to visit my husband's family for Christmas. We were staying with his grandfather who lives across the road from my in-laws because my in-laws don't really cope well with house guests, so they don't ever like anyone to actually stay in their guest room (that's another story!). But they decided they wanted to do Christmas eve dinner at their house instead of my husband's grandfather's house (even though he has a bigger kitchen) just because. Dinner wast at 7pm and our daughter went to bed at 6:30pm because she was sick and really tired. Because we wouldn't keep her up and bring her over to dinner (because obviously everyone wanted a screaming, over-tired baby at dinner, right?), they refused to come eat with us. My husband had to go collect our plates of food and we ate alone across the road! My mother in law thought it was ridiculous that we wouldn't just leave her alone and come across to dinner and spent the evening with them. Literally, she was angry that we wouldn't just leave our sick and feverish 10 month old alone all evening in a strange house to have dinner with them. I was like, um, no?! (as it turned out, she was really sick and vomited in her sleep, had I not been there listening on the monitor, she might have choked and died if she'd been alone!) So thankfully we refused and we ate alone that night and stayed with her, but it did make the point that we were doing things our way and they were a bit more flexible after that.
I think pick your battles. When it's really ridiculous, stand your ground, but find ways to be flexible too when you can as sometimes it really won't make much difference anyway. I'd have your husband deal with it though so it doesn't just look like it's you being difficult.
