Not trying to hurt your feelings, but she will probably not stay with you in the long run if you do go through with it. If the first thing she tries to do with you is sex, then there is no hope of a lasting relationship with her. I just went through a horrible break up, and I don't want anyone to have to feel the way I did when she left me. If you are interested in her, try to grow a relationship over time before doing something so drastic. Give it time to grow into a relationship where you both love each other before you decide to take that next step. Trust me, I know from experience that rushing into something sexual will only lead to failure. Hope this helps man, stay strong. :) Edit: I didn't mean it while you're still living with both of your parents. If at some point you enjoy her company and your step parent is gone, then it may be viable, put a relationship with your step sibling will never turn out ok.