Give Koichirou some slack, he felt bad and guilty about it. He never pursued her or forced her and when his feelings got strong he tries to get away from her. He even moved out. He did try to see her as a sister, which the both of them tried to go for the majority of the series.
Plus to note, Kouichirou was also sorta romantically retarded due to his parent's divorce. He never loved another girl until Nanoka came along and he was already falling for her BEFORE the sibling bomb. Nanoka was his first love.