My girlfriend didn't get fucked, but I came on my step sister-in-law's ass while her panties were on

I had a plan to bang my girlfriend that day, but she had other plans. She wasn't in the mood for sex, and instead, she wanted to chill out on the couch. Being the obedient boyfriend, I didn't push the issue, but I still craved some action. That's when I turned my attention to her sister-in-law, a stunning blonde with a big ass and beautiful tits. She was lounging on the couch, her panties hugging her round ass cheeks. I decided to spice things up by playing with her ass, running my fingers over the material of her panties. Then, I unzipped my pants and started stroking myself, aiming for her ass. With a final pump, I came all over her panties, covering her ass in my cum. It was a close call, but luckily, my girlfriend never found out.