Yori and his twin sister Iku used to be inseparable during their childhood. But lately Yori's attitude toward his sister has changed. He is eager to stay away from her and treats her coldly. Despite his odd behaviours, Yori doesn't hate his sister at all, but is in fact madly in love with her. He no longer sees her as a sister but as a woman. He knows that his love is unacceptable, so he tries to forget Iku by bedding her best friend and even by applying to attend a boarding school in another city. However, it becomes even harder for him to suppress his feelings especially when Iku starts talking about going out and having a boyfriend. Yori can't allow someone to steal his little sister away from him, so he confesses. Iku loves his brother and is willing to do anything to keep them together. But if Iku's love is a love toward a brother, Yuki's love is a love that goes beyond the affection for a sister: a forbidden love.

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