As you and your sister lay there in her room, cum splayed across your belly from where you'd finished after masturbating together, her eyes met yours.
"We can't keep doing this," she said, and a pang of guilt wracked you as you realized the truth of what had just happened. "We can't. I can't keep watching you do this." You knew she was right. It had to stop. It was unnatural. "I need to fuck you. I need to feel your cock buried inside of me. I want all of that cum in my pussy." OK, so you weren't expecting that, but could you really say no?