Otoko Girai na Class no Bijo ga, Jitsu wa Brocon Shitte Shimatta Kekka.

Eiji Mizuno, a high school student, receives the worst possible news from his sister: she has a boyfriend. In order to identify the man who took his sister away from him, he follows her on a date, but a suspicious shadow is watching both of them in the same way. It is Aoi Hasegawa, a classmate known to hate men despite her beauty. And her brother is the boyfriend in question!
So they enter into a joint relationship in order to separate their brother and sister, and end up dating as fake lovers.