You Weren't My Sister, But My Fiancée?!

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!