Otouto no Kyorikan ga Baguttemasu

Hana Yoshida, a high school freshman, falls in love with her classmate Taisei Hagita. But when their parents remarry, Hana and Taisei become siblings. Hana puts her feelings away and vows to become a good sister. But when Taisei starts living in the same house with Hana, she feels too close to him, and every day is filled with excitement...?

(Source: Kodansha, translated, edited)