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