I'm going to call my sister stupid when she refuses to eat
Calling your sister stupid for not eating seems pretty rude. Do you really need to be this mean to her?
Yes, she has done it to me before.
It is mean and rude to tell others how to eat food. You should mind your own business and tell her how you feel when she hurt you.
Maybe you are right. She's not doing anything wrong and I don't care what she eats.
I'm glad you see that her eating habits are harmless. Now whenever she calls you stupid you can tell her the same thing and hopefully she'll be reasonable and back off.