Sister Scheme: Oneechan to no Shikata ni Tsuite

Anjou Sakito is a normal high school student. He is in love with the president of the public morals committee, Kusaka Isami, who is also a classmate. His blood related sister Anjou Marine is in love with him though. One day at school she gets horny while talking to Sakito, and she ends up masturbating in the toilet. Later that day Sakito gets a letter containing a picture of this event and instructions he has to follow if he doesn't want the pictures spread. The first instruction is to take a picture of him kissing his sister in front of the school's gates.

(Source: VNDB)