My friend is really irritating me HELP???????????

ok so i have this cousin well she is not my cousin we say we are cousin coz our moms are like sisters but now she is turning into a pain in the butt...everytime she is around boys she flirts with everyone even with my ex..she knows it bothers me that she flirts with him coz i still like him but she still does it...everytime we are doing something like BBQ or a party with the family and she sees her cousin its a girl sitting alone she goes and grabs her to go and play or do something but when she sees me she just ignore me..but everytime she is alone she always come and talks to me..and know i broke up with my bf a couple days ago and she flirts with him in front of me they give each other hugs and some stupid hand shake and it just pissed me off..like i really want to fight her throw a fist at her face coz she is irritating me but i dont want my mom and her mom to start arguing like what should i do..( i known her for 2 yrs now)