I kinda wanna slap my younger cousin really hard on her face.
Why would you want to slap your cousin?
She's always trying to look like me and I take it as an insult.
You shouldn't attack your cousin. It sounds like she is trying to emulate you, but if it bothers you, you should talk to her instead of slapping her.
I'm going to slap her hard on the face until she stops doing it.
Imitation is the sincerest  form of flattery. It seems she looks up to you. Still you have problem with her emulating you, just talk to her, you shouldn't resort to any kind of violence.