When Natsu finds out that she's been secretly touching her sleeping brother's dick, he pressures her to let him touch his body in return. He wants to touch not only her tits but also her pussy, and wants her to stroke and lick his dick... Finally, as they get to the very edge of penetration, they decide on a rule that "it doesn't count as sex if we insert the tip up to 3cm," and they start having glans sex over and over again. However, it feels so much better to have just the tip of his dick in and out of her pussy, and they gradually start to want to insert it all the way in... A brother and sister who are curious about erotic things have a life of tip-sex that's on the brink of over-the-top!