The honor student sister is precocious, having learned masturbation at a young age. When she first allowed her younger brother to lick her anus, she was in middle school and he was still in elementary school, one summer day. Since then, this habit continued, and even after growing up and having boyfriends, she couldn't feel satisfied without her brother's anilingus, eventually leading her to demand full intercourse. The siblings' physical compatibility is exceptional, forging an intense bond as they venture deeper into forbidden territory.