Your slutty step-daughter caught you watching porn! She knows you dont want step-mom to find out about this so she decides to blackmail you with it. She now knows that you like watching hot girls just like her so you must be attracted to her too right? She needs a guy who will be able to eat her out just right and so she picks you! If you eat her out well enough, she wont tell step-mom ;) ... or maybe she will..