Nanami couldn't bear to see her brother unable to masturbate because both his hands were injured, so she helped him stroke his penis. Since then, secretly supporting him in masturbating has become a daily routine. It doesn't end with just stroking with her hand, they also do blowjobs and handjobs... Finally, the two of them come close to penetration, and they decide on a rule that "it doesn't count as sex if we insert it up to 3cm from the tip," and they start having incomplete glans sex. Is 3cm insertion just about safe? It's erotic to see the brother and sister who insert it deep even though they know it's wrong! The guilt and pleasure when they insert it all the way to the back of the vagina are exceptional!