OH that explains some stuff!
Where I'm from, people like that are referred to as 'supertasters' or some variety. It means your taste buds (or whatever is in your mouth) are so sensitive certain foods and textures can make a person gag.
Where I am from, a picky eater exclusively means "someone who chooses to be difficult about food". That's why I hate picky people about anything- where I'm from, "picky" ALWAYS means "choosing to be difficult and to make other people's lives difficult".
Supertasters? huh, and yes taste buds would be the problem. Want to know something really freaky and weird about me my doctor cant even figure out? I cant drink water. Why? Because to me it tastes like blood.
I'll elaborate. You know how blood tastes like rusty metal? Well apparently no matter how filtered the water is, i can taste metal ions in it and to me therefore water tastes like blood. My doctor thinks its fascinating, to me its a freaking pain in the ass.
And i see you have some issues with your sister and her food choices, but maybe she just really doesn't like the food. Personally i feel being a picky eater is a real horrible burden, unless you are one, its horribly hard to understand just how difficult it is to go out to eat at certain places. Like for me, (not that i like expensive places to begin with) but they at fancy places they ALWAYS have to make food extravagant with lots of weird ingredients stuff doesn't need but to everyone else tastes wonderful, but to me its uneatable.
Like the other weekend. For Easter my family went out for a stake dinner. We ordered what we thought was just this type of three cheese bread. Turns out the bread had raisins in it. So i tired picking them out... but i could still taste them in the bread and couldn't eat it. Then out stake came with some kind of mushroom side stuff... and (as childish as this sounds) it was touching my potatoes, so i couldn't have half my potatoes.
Honestly i wish i did like more stuff just dont, and when i was younger i would get really upset when i went out places and had to suffer through only being able to eat half my meal, or asking for things plain and technically getting less food, or having them mess it up add something on anyway and then me not be able to eat it. Maybe your sister isn't just being difficult maybe she just really doesn't like it?
It can really drive a person mad! One time i was in London. It was my second day there. And I hadn't been able to eat the day before because of all the running around to get to the hotel and such. And that morning Will and our friends decided to go off for breakfast without asking me (I was so pissed) anyway Lunch finally comes and we go off to this small cafe place. I get a cheeseburger like usual, which i asked for plain, only i didnt know if that means something different over there but it came out with ketchup and mayo on it. i tried to eat around it but couldn't... so I just lied and said my eyes were bigger then my stomach, and waited for Dinner. Sadly dinner was at this banquet place our whole school had to go too.. and my stupid school decided "I bet there are probably as many vegetarians as non-vegetarians so lets order the same amount of food for both." Sadly that wasn't the case and they had this like table order thing and i was at the last table, so i only had the choice of vegetarian lasagna.
IT was a day from hell!!!! Point of my long rant is maybe you shouldn't be so hard on your sister? Unless she is just doing it for attention or something. But being a picky eater is actually a real burden.