A small-breasted little sister and a big-breasted big sister. One day, the little sister realizes that her big sister has super sensitive nipples. She sneaks into her sister's room, where she is secretly masturbating with her nipples, and licks and sucks them! Her little sister's tongue technique makes her cum in no time, turning her nipples into weak ones! From then on, she targets her big sister's nipples at every opportunity! She plays with her big sister's nipples while laughing, developing them every day! The big sister becomes so sensitive that even the slightest stimulation makes her areola cum, but at the end, she ends up licking her little sister's nipples...