How wrong would I be to tell my sister in law to fuck off
Why would you treat her so rudely?
I'm just acting like that because I've had enough of that bitch.
Family members can be annoying, but if you tell them  to fuck off then it'll just make things worse. They'll further have no reason to avoid annoying you and may even find enjoyment in doing so after you insulted them.
She's not going to stop pushing me anyway.
She'll probably keep pushing you until she doesn't get the reaction she wants out of you. By losing your temper and cussing at her you're giving her exactly that. The best way to handle this is just walk away when she's pushing your buttons.