Otouto no Koibito ga Mukashi Choukyo shita Motokano datta

Kazuki has finally managed to start dating his crush, a cute girl in his class named Aoi. It's a dream come true! What he doesn't know is that when Aoi was younger, she was seduced by Kazuki's older brother, who took her virginity. Once Kazuki's brother realizes what's going on, it doesn't take him long to exploit his lucky break. He kept some sexy pictures from the tryst years ago to blackmail Aoi with, and eagerly starts to cuckold his unfortunate younger brother.

(Source: Irodori Comics)