Omae Imouto ja Nakute Iinazuke Datta no ka yo!?

"I wonder when my sister Kana started becoming distant from me—"

Shina Takashi lamented his current days, where being ignored was not uncommon. One day, his parents dropped a shocking revelation. Not only was Takashi not their biological son... but he was also the fiancé of Kana, whom he thought was his sister! From siblings to potential romantic interests, it's not easy to sort out feelings. The fluctuating emotions, the bewildering distance. Their relationship undergoes an inevitable change—! She's a sister but not a sister!

(Source: Kadokawa, translated)