The brother noticed his younger sister Aoi's suspicious behavior. When he confronted her, she said she had been practicing putting on a condom with her brother's penis. Since then, he has been helping Aoi with her condom practice, but it's too painful to leave an erection unattended, so he asked Aoi to help him ejaculate. He couldn't bear it with just handjob and even went as far as fellatio and cunnilingus... Eventually, the two of them got to the point where they were about to insert, and they decided on the rule that 'if it's just 3 cm of the tip, it doesn't count as having sex,' and they started repeating the glans sex. Before long, just inserting the glans was not enough, and they both began to want to insert each other's vaginas all the way to the cervix.
