OP, I definately agree that this is something I'd want to be informed of if I were a guest, but if I were DH's Cousin, I'd really appreciate it if you brought your concerns to my attention first.
Granted, DH's Cousin should absolutely have informed her prospective guests of her intentions when she invited them, (I love high dudgeon's wording) but when your Aunt was telling you about her wasabi taters, you might have said "Wow, those sound pretty tasty, but do you think MIL and FIL are going to be able to enjoy them? You know how sensitive they are to spicy stuff." That way, you've given her a polite prod to discuss with Cousin her guests' needs and opened up a possible discussion about non-spicy alternatives.
It would also make it seem a lot less...subversive (maybe? I'm not sure that's quite the word I want...it sounds too harsh) when you provided MIL with a warning and an invite. If you've let her know about your concerns, DH's Cousin would be hard pressed to view your MIL & Co ending up at your mom's dinner as anything other than legitimate concern for their comfort. (As opposed to an "Ewww, Gross!" reaction)
Edited because I can't spell today.