I know this threads been sabotaged, but honestly I know two couples who got together because their parents got married and they got it off. There's nothing wrong with it and, in England at least, there's no law against it: you're not genetically related. The queen and prince Phillip are closer related than you and your step sister. If it's going to make you feel awkward then tell her to stop it, if not then blam the blam out of her. If you get my meaning.