People say it takes "two to tango". In your case, it seems like it takes three.
It also sounds like you're very young. Your bf's cousin maybe wouldn't have started hitting on you unless he thought you wanted him to.
The 'advantage' he's taking of you is of your youth and inexperience. From the picture you've painted, it sounds like they're both jerks and you're a bit of a bozzo, too.
Get away from both of them and give yourself a chance to grow up under your own terms. An engagement ring is no guarantee of anything, apart from (perhaps) a life of misery.
You're young enough to learn from your 'mistakes'. Sex, drugs and house music (whatever) are only ok in the short-term. Good luck.