She is being selfish. If she just came down with symptoms yesterday, and it’s really noro (how does she know?), then she really won’t be in a good place to do such a trip, and it will affect all of you one way or another, whether it’s passing along the germ to the rest of you, or not being able to keep up because she’s feeling bad. Sick? Stay home, pls.
I picked up noro once at work, and man, I felt like I wanted to die for a couple of days.
I remember going to WDW one January when there was a noro outbreak. My kids were just little, and we debated cancelling. But it was the only time my DH could take a vacation, so we went, but I schooled everyone on how Noro was an oral-fecal germ

(with some evidence that it can also aerosolize during times of things like toilet flushes), and that, whatever we did, our hands had to be clean before we touched our faces or mouths. We used lots of hand sanitizing wipes. And somehow we never got it. Same with on four Disney cruises during times of noro. So it can be done, but there’s no guarantee (like, I got it at work, from the bathrooms, I’m sure).
It’s a known fact that sharing a bathroom with someone with a GI illness puts others high at risk for catching it, too. Being outside, I think, is a bonus in this case, though, for transmission. Is it possible for you, if you go, to create your own “bathroom”? And eat just your own food supply? At minimum, she should not be touching shared utensils and if possible, use just her own bathroom. But that all just seems ridiculous/not really possible on a trip like this. They should really both opt out this time. Personally I would probably still go even if they go, but I’d half expect we’d be calling a Med Evac at some point in the heat and exertion. Hope her insurance is up to date!