Kimi no Iru Sekai Shikairanai

After his mother's death, Kazayuki goes to live with his younger step-brother, Futaba. Unlike Futaba, who is delighted to know that the kind and pretty boy who helped him when he cried as a child was his older brother, Kazayuki seems cold and unfriendly after their reunion. Their feelings continue to be at odds, but one day after school, Futaba whitnesses Kazayuki having sex with a girl. It makes Futaba realize that he loves Kazayuki, but Kazayuki's next words to Futaba are..??