Because I Don't Like My Big Brother At-All--!!

Nao Takanashi has always loved her brother Shūsuke, even though society frowns upon such feelings. She believes they are blood-related, but later discovers she was adopted after her birth parents passed away. Realizing that adopted siblings can legally marry in Japan, Nao decides to pursue a normal romantic relationship with him. But she soon faces competition from Shūsuke's childhood friend, Iroha Tsuchiura, and his class president, Mayuka Kondō, who has a fondness for yaoi.