Sakura Iro no Amour -Imouto Amour Hen-

After their parents passed Kazuki and Ayano had only each other.
But when Ayano confessed she loved Kazuki as more than a brother,
he was spoken for, and gently rejected her.
Over time, Kazuki feared he would lose his sister to another man.
By the time he realized his own feelings...

... it was too late.

[From DLsite English]