I'm planning to call my sister a fat bitch after she ate all the nice food
Is there something that you could do instead?
Actually, I can also stop my sister from eating that nice food during lunch
I think that there are much better ways to go about this. Calling someone an insulting name is not the way to go about it.
Maybe she'll lose some weight if I call her that.
It's understandable you have strong feelings toward your sisters weight. There's better ways to go about expressing this to her. If you truly care about her weight you should talk with their and help her with ways to lose the weight.