Since we're all speculating wildly, I suspect little sister is maybe not thrilled to be at home, also. Like, "Hey you've got your whole condo, why are you here being too much" - I imagine that kind of privacy seems like a luxury when you're quarantining with your parents (and vice versa if you live alone).

I am definitely also in the camp that the pickiest eater should do some of the cooking, if they can. Like, this is probably the best way to share with your parents food you actually like/can eat, you know? Plus it's just a gracious thing to do even if you are technically the guest (and might have to fight your parents to cook when you are the "guest" -- if their family is like mine)