my older sister, who loves sex and calls me "virgin-kun" and makes fun of me, treats me equally even though I'm surprisingly kind and introverted. one day, by chance, she even cums inside me with handjob! after that, i became my older sister's sex slave? eventually, i was even made to have sex... my older sister tries to train me as a friend with benefits, and as we repeat sex many times, i end up wanting to have sex without a condom that i had been wearing until now...