While his girlfriend was away, Natsuki Rin secretly had sex with her younger sister, who had a strong sex drive and premature ejaculation, during their five-day cohabitation. (FSDSS-303)

My girlfriend's younger sister, Ling, was coming to stay with us for five days. I accidentally saw her naked, and it was unbearable, so I sneaked into Ling's room while she was sleeping...