Giri no Imouto to Kekkon Shimasu

Synopsis

When my father remarried, I suddenly had a step-sister, and I fell in love at first sight. That was what I, Takuro Matsubara, declared at our first meeting. "I might look at you, Toko Konoe, in a sexual way." For some reason, my step-sister's first impression of me was terrible, and our life started with me having a negative-infinity favorability rating. (Source: MF Bunko, translated)