You walk in on your step sister, and she's practically naked in bed, playing video games. You just "came in to say hi", but you conveniently made sure not to have a shirt on. You knew what you were doing. You'd been curious about her for a while, and you wanted to get a taste for yourself. At first, she's kind of grossed out, but slowly you get her into the idea, and the next thing you know, she's sucking your rock hard cock, and you're pumping her full of your hot, creamy load. This might be the start of something... ;)