screw black mail, screw leverage, if you actualy care about your dad tell him now, it's going to come out sooner or later, no matter what cuz no one manages to get away with it forever, atleast this way he knows he can count on you, and you know you'd put your dad's feelings/emotions over a drug, i would however, torture her with the fact that you plan on telling your dad at the first available oppurtunity (i.e. he's at work, she's at home, you walk up to her and tell her you know, and when your dad gets home, so will he, and there's nothing she can do to stop it, then add in a bitch comment lol)