I have never heard of artificial sweeteners being in so much before, but then I love sweet n low, so I would probably never notice something like that.
But, if I was highly sensitive to this product, I would most certainly be asking about it being in my food when going out to eat at any place other than my own home.
I don't think any restaurant can possibly list all possible allergens on their menus. At this time, it seems to me that I see the most common allergens listed on menus, labels, etc., (milk, soy, nuts, wheat, gluten, etc). I have never seen artificial sweetener listed anywhere.
Like another poster, I don't see this situtation being the restaurant's fault. Your sister could have avoided her sickness by asking that one important question before she ordered her food.
However, no matter what the reason, usually when someone gets highly ill after eating somewhere, they just cannot eat at the place again or that type of food again. My dh got food poisoning one year on shrimp scampi. He has never touched the stuff again. One Christmas holiday season, he got sick from a virus, but had eaten Kentucky Fried Chicken and Subway that day and he has not eaten from those places either as of yet.