I'm going to yell at my sister for making fun of my girlfriend at a party
Does your sister make fun of you as well?
She did it to me once and I'm not okay with that. I told her I don't like what she did.
It can be upsetting to be have your girlfriend be made fun of, but you shouldn't yell at your sister at the party. It will embarrass her and it will upset everybody who is watching.
No, the party's over, so I'm going to yell at her about it anyways
The harm that your sister has caused to your girlfriend has already happened, so you should try to stop it from happening again rather than getting revenge.  If you yell at your sister, she will just get mad and might lash out again in the future.  But if you calmly let her know that her actions were inappropriate, things could get better.