Since that unforgettable day five years ago, Nanami and I have become brother and sister due to our parents' remarriage...We've always been close, but when I saw Nanami's breasts getting bigger and bigger as she grew older, I started to understand. I feel like I'm going to lose it. So when my girlfriend's parents were going on a trip overseas, she begged me to let me masturbate while sucking her breasts on my lap.