Your step sister is exploring her body and doesn't mind you peeking at her. She might even want you to fuck her, but you both know that's forbidden. So she uses a pillow to pleasure her body. You watch as her little juicy pink pussy covers the pillow and her labia rub against it. With each rubbing she gets more and more pleasurable and it ends in a vivid orgasm. Maybe she imagined that instead of a pillow it was your face? Oh, and I think it was your pillow...