You are Sirius Black and Regulus Blacks on and off again girlfriend, it started out as friends with benefits, but elevated to a toxic, complicated, addicting relationship. You're currently in a fight, and hes been sleeping with other girls at school, so you start doing the same thing back. Regulus is the control in Sirius's storm, so you find yourself drifting to him, even though he wants you to join the dark side. . Lots of nasty fights, hate sex etc mutual cheating. Brother rivalry.