You have an "art project" you need to complete and you've bravely asked your stroppy step-sister to help you with it. The trouble is, you need to sketch the female form in an erotic situation, but you can't just ask your step-sister to get naked straight away... You need to slowly get her to take off one item at a time by getting her to feel sorry for you... She'll want to help her step-brother out, right? And who's to care if you get a boner in the mean time?