Ai Shimai Tsubomi... Kegashite Kudasai

Katori is attracted to an enigmatic new student at her University named Koshiro. She remembers his face from when he saved her younger sister from a band of rapists in the past. However, rumours abound that Koshiro is sleeping with his teachers, and has some kind of hidden agenda against the school and her mother. When Koshiro's plan goes into motion, Katori, her sister, and her mother will all be in on the action.
(Source: Anime News Network)