If you are really interested in the welfare of your cousin, do this:
Have a tape recorder handy and tape her advances. Then fuck her. Take a piece of evidence like her panties, tell her you're the kind of guy who is into that kind of shit.
Then go to your cousin. Play him the tape so he understands she made the advances, not you. Then show him the panties, which is evidence to show that she wasn't merely flirting. He'll dump her for the double dealing whore that she is, and you get to have free sex. He'll be mad at you, but later on he'll be happy that you called her bluff and exposed her true character. And she'll really learn her lesson because she got betrayed herself while betraying, and won't be able to pin the blame on you because you have the tape.