One of my worst pet peeves is people saying "ugh, that's so retarded" and "you retard"

My younger sister (12 years old) uses this all the time. I keep telling her that's it's a serious condition and she turns around and says it again. It makes my blood boil. I can't get it into her ignorant skull that it's offensive and on the occasion that I say that, she doesn't care at all. This ignorance is horrible. Does anyone have a way to tell these people to stop using the word offensively?

Thank you