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

Nao Takanashi loves her brother Shūsuke despite the incest taboo. Nao thinks that Shūsuke is her blood relative, but in fact she is an adopted daughter, whose parents have died. After Nao learns the fact, she wants to fall in love normally with Shūsuke because siblings by adoption can marry under the family law in Japan. However, she finds herself competing with Shūsuke's childhood friend, Iroha Tsuchiura, and his class president and yaoi lover, Mayuka Kondō.