But something else was also happening. The previously mentioned precarious nature of the fly of my boxers meant that, as I humped into my sister's ass, my penis was starting to pop free. I could feel the fabric of my sister's panties tickle the skin of my dick with each press.

Lucy must have felt it too. She reached back with her hand, stopping our coupling. I thought, once again, that she was drawing the line.

Instead, she pulled her panties down her legs and tossed them off. She lay back down on her tummy.

"Lucy, are you sure?" I asked. The argument roiled inside me; I could almost picture...