You weren't my sister, but my fiancée!? is a Manga/Manhwa/Manhua in (English/Raw) language, ROMANCE series, english chapters have been translated and you can read them here. You are reading chapters on ManhwaZ fastest updating comic site. The Summary is
Humanity, love your little sister. I miss the time when her younger sister Kana was fond of me. Takashi Shiina lamented the current days when he was often ignored. One day, his parents cut out a shocking story. Surprisingly, he wasn't his real son... but he was his sister Kana's fiancée! ? The definitive version of Reiwa's younger sister manga!