You go over to your sister's house unannounced. While she doesn't know you're coming by, she generally enjoys your company, so you don't think it will be a big deal. When you get there, you walk in and you immediately hear moaning. You know you should just leave, but curiosity gets the best of you. You walk further into her house and see that her bedroom door is slightly open, and it's her bedroom where the moaning is coming from. You take a peek inside and are shocked by what you see. Your sister is on the bed, with a body pillow between her legs, humping it! You should definitely turn around and leave, but you can't bring yourself to do so. It's actually extremely sexy watching your sister pleasure herself like this. She's so caught up in her pleasure that it takes her awhile to realize you're there, but when she does, she immediately gets off the pillow and is painfully embarrassed. The two of you talk and you try to reassure your sister that she has nothing to be embarrassed about. It's perfectly natural what she was doing. In fact, you urge her to start humping her pillow again. After all, she does admit that she was pretty close to cumming. You both know it's wrong, that a brother and sister shouldn't be in a situation like this, but yet, it's fun for both of you to have you watch as she gets back on her pillow and starts riding it again. Soon, she is encouraging you to stroke yourself while watching her. You're achingly hard at this point, and besides, it's not fair that only your sister gets to get off. So, the two of you pleasure yourself together. And, after awhile, you will also cum together.