Now I knew that my sister wasn't a virgin; in fact I had to help her out of a jam with an ex a year before and she confided in me about the entire relationship. But the fact that she was really enjoying this, her hand stroking up and down on this kid's surprisingly big cock really kind of shocked me. And turned me on. The look of animal, sexual need on her face was intense. And I finally got to see her perfect pair of tits naked and aroused. Watching her beat this guy's meat as if her life depended on it was just killing me. I knew I...