Aki-Sora: Yume no Naka (2010)

Aki and Sora Aoi are a pair of close siblings who have shared an intimate bond since childhood. During their coming of age, they each come to realize the true depth of the love they feel for each other and consummate that love in secret. They keep Sora's twin sister Nami in the dark about their new relationship. Nami, clueless of what is happening in her household, aims to set her brother up with her best friend, Kana, for whom Nami herself holds her own secret feelings.