Some people just aren't interested in cooking. I was very interested in cooking, started when I was probably 6 just stirring crackers in a pot and then eating them. I still cook...a lot...its one of my favorite things to do. Others, not so much, so if they aren't interested then they likely wont try. If your sister just wont cook for herself and it's something that's always just done for her, show her what to do and then if she still wont, she'll just be hungry. Most people in the US wont starve due to lack of food, they'll get hungry, but wont starve. If she gets hungry, maybe she'll actually put in the effort to cook herself something.
That said, there are grown adults that never bothered to learn to cook, and they eat takeout and fast food every day of their life. Only cooking they ever do is microwaving a lean cuisine frozen dinner.