I'm going to mock my friends sister
Do you have a problem with your friend's sister?
Yes, she's so annoying and I'm sick of her.
I understand she is annoying to you but you shouldn't mock others. You just need to know her better in order to tolerate that quirkiness she posses.
I don't care about getting in trouble with her. She has no influence.
You don't need influence for people to defend somebody. That girl could turn out the nicest person you've meet. So, give her a chance.