I'm so annoyed at my sister. I'm going to tell her to knock it off
What do you need her to knock off doing?
She talks to much.
Knock it off comes off as really mean. You should try to have a nice conversation with your sister about what is bothering you.
But why do I have to be nice and talk to her.
Because she won't listen to you if you're being mean. People are more willing to do what you want when you ask them nicely to stop.