Suddenly, Yuu came to my house after my parents remarried. We are not related by blood, but we are brother and sister... We try to suppress our feelings for each other and not cross the line between brother and sister, but... The brother gets excited after seeing his sister masturbate and has sex with her for the first time. He even makes her give him a deep blowjob in the bathtub, and at the end, we have a threesome with his sister's boyfriend!