My arrogant younger sister gets a boyfriend, so I, her older brother, have to do NTR and become her sex slave. (Winter Aikoto MKON-016)

My adorable little sister, Kotone, used to say she'd marry me when we grew up… Lately, she's been avoiding me. I thought she was just being tsundere, but it turns out she has a boyfriend… So, I guess I'll be her first boyfriend…