Hifumi fell in love with Miike at the train where they takes the same line for school. From that time he knows that Miike fell for his sister. When Miike handed a love letter with written wrong name to her, so Miike mistaken by her and she gave the letter his Hifumi nii-san. And I believe nii-san thought it was one in a million chance, that's why he went for it and didn't allow Miike to correct the mistake. They spent time together here and there, eventually Miike started to have feelings for the brother.
Kimi Towa Gokai Kara Hajimarimashitaga