From allcomic.noor.jp:

Masaharu Banaga lived alone with his elder sister, Akira, due to his parents' circumstances. Akira was a member of Sweet Mints, a popular idol group beloved by Masaharu's classmates. One day, Kazuki and Misuzu, members of Sweet Mints, came to Masaharu's house and started drinking with Akira. It was then that Kazuki and Misuzu revealed that his sister Akira was not his biological sibling, but an adopted child. Though shocked to learn they weren't truly brother and sister, Masaharu realised they could potentially marry. This discovery made him become even more preoccupied with his sister than before.