Lil step sis was trying her new yoga pants & she was in doggy position as she was trying some exercise when I walked in. She didn't notice cause she had headphones on. I was watching her & conflicted. After all, she's my step sis! Damn I admire the view in front of me! So I come up with the plan to try & wank one quietly. I undo my pants & begin to jerk myself off while staring at her ass. She accidentally grind right into my cock! She turns around in shock & tells me "What the hell do you think ur doing!" She then start to tease me about the fact that she's my step sister and how taboo this is! She finally give up saying something like 'fine do whatever you want, just don't cum on my pants! I take the helm at first and grinds against her clothed ass cheeks dry humping her yoga pants. Eventually she takes full control and are doing most of the grinding, rubbing her ass all over my cock. Eventually I cum from the sensation of your ass over my cock - no hands or anything just from the frotting alone.