I grabbed my cock, and started to jerk it, thrusting my hand back and forth. I quickly get into a rhythm with it, and start to gently squeeze in and out along with the back and forth motions. Even with the panties down at my crotch, I can still smell her pussy juice on it, and as I take in her aroma I can feel my cock swell larger, which prompts me to squeeze down harder on the tip of my cock each time I reach it. I start to punish my cock with harder, rougher jerks; between her glorious scents, and the pressure the elastic of the panties on my balls, I start to feel pre-cum leaking out of the head of my cock. I took it and rub it over the length of my shaft and continue to jerk, spitting onto my hand to increase the lubrication. By this point it's so moist I can close my eyes and imagine that there's a mouth around it. My breathing starts to get a little harder, so I take my free hand and use it to lean against the wall; it's been awhile since I've cum, either alone or with a partner, so I know this is going to be a big one.
Part of me wants to slow down, and enjoy the experience more, but I'm feeling greedy, and just want to finish myself off as quickly as possible. I jerk harder and faster, harder and even faster, As I feel myself starting to approach orgasm, I take the panties and pull them back over my cock, putting my hand down the waistband and jerking from inside them. A moment later my cock exploded, cum squirting out all over the fabric. There seems to be an endless supply of it coming out, and the fabric of the panties turns so dark that I don't know if I'll ever be able to get the stain out. Which, I guess, means that these panties are goign to become mine permanently and I'll have to tell my sister-in-law that they got lost in the wash.
"That should just about do it," I heard from behind. I turn around to find my sister-in-law standing behind me, video camera in hand. "I think we find ourselves in an interesting situation here," she said to me with a smile.