If you don't like her advances, and if it's unwanted, then you tell her to stop. If she doesn't stop, then she's sexually harassing you (Don't kid yourselves, folks. If the genders were reversed, if OP was a girl and it was her step-brother doing it, you'd all be saying it's sexual harassment. Don't hold a double standard) and you need to let someone know, like your parents, a school counselor, or even the police if need be. But let's say you reciprocate the "feelings" and may want to indulge yourself, I'll tell you why it's a bad idea. What do you think is gonna happen when your parents find out? And they will find out, one way or another. Or what if she tells them about it first? Whose side of the story do you think they'll believe? It ain't gonna be yours.