Kissing Question about me, my boyfriend and my older cousin Sammie?

Me and My boyfriend kiss each other with tongue and on the lips and snuggle too.Dating for 5 years I am 12. My older cousin Sammie is 12 and a half and came in and started basically being a smut. In undies and a skin tight shirt no bra. She flirted with my man and made me really really pissed off. So I punched her and broke her nose. Then me and my boyfriend kissed a lot and took a nap on the Gray futon and she was mad and tried to screw ma boyfriend what should I do about her? Kill the ***** or let her hurt him?