Manga Like Jitsu wa Imouto Deshita.: Saikin Giri no Otouto no Kyourikan ga Yatara Chikai Wake

My stepbrother, Akira, was introduced to me as a high school student when our parents remarried. He is a beautiful boy but shy because he grew up in a difficult family environment. I've always wanted brother, so I spent time with Akira for his sake, and...... we hit it off right away, and we quickly grew close! I asked him, "......bro, do you like me?" "Yes, of course." Our relationship deepened despite a critical misunderstanding. Until finally, I realized that Akira was actually my "sister" and I became very confused....... "I liked the relationship between us when we were just brothers. But I wish we could be something more than that." Akira's new approach, trying to make us "lovers" instead of "siblings," has begun! A flirtatious love comedy with an easy-going, too-cute, little-brother-like sister! (Source: Fantasia Bunko, translated)