I've decided that I'm going to yell at my cousin.
For what reason would you want to yell at your cousin?
I get mad at her for looking like she knows everything
Yelling at your cousin is not nice. You need to find another way to release your anger.
I know, and ultimately I'm the one who caused this. Yet I still think I should be able to yell at her.
Yelling at your cousin is rude and it's going to accomplish nothing aside from creating more conflict. Ultimately you won't feel better and you'll hurt her feelings in the process. Have you considered taking a walk or removing yourself from the situation in order to give yourself a bit of time to cool off?