Phantom mother, forbidden mother-child swap, obscene ritual of a big-breasted female family

A tradition that has been passed down from generation to generation in a big-breasted family is that if the baby is a boy, that is, a son, the mother uses her big breasts to steal the son's virginity. Naho and Rin, two busty sisters who learned this legend from their mother, Fumie, each struggle to catch up with their son, but they are having a hard time... One day, the sisters return to their parents' home for the first time in a while, taking their son with them. There, Fumie hears from her grandchildren that she is still a virgin, and decides to perform a certain ritual. It was that Naho was engaged in obscene acts with Rin's son, and Rin was engaged in obscene acts with Naho's son. Then, it finally develops into a mother-child swap with the four of them mixed together...