She just moaned in response and I stared in disbelief as my glistening shaft disappeared inside her and then slid smoothly back out. I watched open-mouthed and shook my head. This can't be happening! I thought. It took another minute for me to realize it wasn't Karen thrusting her hips insistently back toward me as I had assumed. It was me, driving my hips forward and f*****g my sister in the ass.
Then it felt like something snapped into place inside my head and I shuddered as that realization shot through me. We both wanted this, and had for some time. Despite my feelings of guilt, I don't know if I had ever been harder than I was at that moment. Karen obviously wanted this—she and Sherry had set it up, after all. My sister had been waiting for me with her asshole lubricated and ready fo
