Part One:

You hear your step-sister in her room moving around a lot, obviously doing something weird. So you go and peek in her room and see that she's taking nude selfies! When she sees you standing there, she invites you in to get better shots of her. While she starts posing for you to take them, you think to yourself maybe she's doing this for you. She is really slutty. Finally, before you even realize you have it, she notices that you have a boner. She then offers to take care of it for you..

Part Two:

 After the other day with your step-sister, which you've made sure to keep pics on your phone of her, you come into the kitchen and she's there. You walk up and start groping her, but she's just not into it and pushes you off of her. You pull out your phone and start showing her that you have the pictures still, and it becomes clear what she's going to have to do for you.