You finished with the most important point: this is your husband's family, so really it's more his issue. And although your feelings are valid and I see your position, you are pretty far down on the list when it comes to decision making this time. So I wouldn't call it a "bait and switch," just evolving plans being made by people who, this time, are in a more central role than you are.
I think that if MIL is fine with her son-in-law's family joining in, that is what matters. She may not have gotten to know her daughter's in-laws yet; well, maybe she wants to. I sure would. I know many families that feel like family and include each other in holidays and gatherings across one or more marriages. Sometimes some people say they feel like the annual beach week or whatever is getting a little hijacked by, say, their sibling's in-laws, but usually it's still worth going. It usually happens because of geography; otherwise the affected sibling and spouse would have to choose between their families or each only go to their own family's event, which is evidently working for this OP, but wouldn't for everyone. Lots of people really like their in-law family and hate to have to choose.
OP, if your husband is fine with you not going, and you're sure this is what you want, then you are all set. But if (and I do mean if -- you don't have to!) you love your MIL and want to celebrate her big birthday with her, then maybe it's worth going. You were planning on it before you knew that "strangers" might be there, right? How much would that ruin something you would've liked to do? Maybe they won't even come, or maybe they will and you will really like them.