Omae Imouto ja Nakute Iinazuke datta no kayo!?

Synopsis

Takashi Shiina longed for the days when his little sister Kana adored him. To his consternation, these days he's more likely to be ignored instead. Then one day, their parents dropped a bombshell revelation. Not only was he not their biological son… but they are also asking him to get engaged to his little sister Kana!? A new kind of imouto manga for the Reiwa era! (Source: ComicWalker, translated)