Having sex in the bathroom with my cute stepsisters/adopted sisters while they're all in school uniforms! Why?? HUNTB-400

I was cleaning the bathroom with my stepsister when her uniform suddenly got soaked, and the glimpses of her body and underwear caused me to accidentally get an erection. "This is bad!" Just as I panicked, my stepsister also seemed shy and awkward, creating an awkward atmosphere. Then, with teary eyes, she rapidly approached me, leaving me bewildered. Wait a minute... Her hand reached towards my crotch. The unexpected erection became the trigger, leading to a secret between my stepsister and me that we absolutely couldn't tell our parents.