I know what just what you mean... I have a SIL who's priorities are definitely different. My nephews were just germ factories, bless their hearts, and ANY time we got together - which was fairly frequently, due to somewhat monthly get-togethers at my mom's. It was literally like SHE wasn't going to deny HER children the experience, and everyone else be darned, but I know her, and I know that's not really her attitude? I think it just didn't register for her. We parent VERY differently, from nutrition to discipline to... you name it. But she's not a thoughtless person. She's just on a different wavelength, where if the boys weren't sick enough to keep home (and they did show up acting much like your cousin's kiddo a few times) in her mind, then there was no reason to say anything to anyone. So we'd show up, and sure enough, there they were, sick and coughing and snot everywhere. It was really frustrating, because of course I want my kids to spend time with their grandparents and family, and they love their cousins, but I was feeling I had to choose between health and relationships. And it wasn't just the dinners - I'd take my kids over there when I knew my nephews weren't there to visit, and they'd show up. Sick. Mom would be watching my kids so my SIL would drop hers off with Mom too, "so they could play" and they'd be sick. Yes, I spoke with my mom repeatedly about it, and if she knew the boys were sick she'd let me know, but it was a long all too diplomatic journey to get my SIL to realize that for us, and for me with my weak lungs - my kids got over things pretty easily but I'd catch it and be down for days - it was more important to stay healthy than for the kids to play together. Winters were miserable, but in the summer we were usually fine.
I started with calling or texting to "see how they were all feeling" since WE had all seemed to get sick after the last dinner. Then if my kids were even a little bit snotty, or whatever, even if I knew it was just allergies, I'd let HER know beforehand, so she could make the choice of whether or not to expose her kids to mine... it took awhile, but she did start letting me know if her kids weren't feeling well from time to time at least. Then we ended up moving so far away it didn't matter any more, and I kind of miss their snotty little noses, haha.
But... like you said, it's only occasionally. Maybe you could call and see if the birthday boy is doing any better (if you could do it with sincerity?) and start a conversation about it like that? In that context saying you all got sick later wouldn't be so accusatory and maybe they'd apologize, or better yet, realize what a bad idea it had been to go ahead with the party? I sound all magnanimous now, but there were a few times when I was just so mad that we were sick AGAIN when all I wanted was dinner with my parents, yk? It took quite awhile (and probably not having to worry about it anymore) to really get to where I could see that my SIL was just flaky in that way, quite possibly desperate to get her boys out of the house (they are a serious handful) and meant no harm.
It's so hard when they're so sick and so tiny like your LO. I'm sorry you guys got ambushed by such a nasty bug!