What would you do if you had the same problem?

my cousin is a bigggg flirt and she's a bit pretty. she brags abut how alot of guys like her. well a few days ago she began to talk with a guy i like. she doesnt like him I KNOW it's just that she still flirts with him and she is also flirting with my other friend mario. well i just dont know what to do. she acts really stupid around people and she is REALLY obnoxios. so what do i do?