Nee-chan no Susume ~Onee-chan no Itazura Seiseikatsu by Atelier Kaguya Bare & Bunny (Jap)

The protagonist Natsuno Shinji spends his days helping his sister, Ai's coffee shop while attending school. However, the situation changed completely when he became friends with a popular school girl, Matsunaga Shigure!

Natsuno Ai, an extreme brocon and Kamiya Aoba, his childhood friend misunderstood Shigure to be Shinji's girlfriend. To take back Shinji, they start teaching him naughty things.

With his mother-in-law, Utako, and Shigure, what would the become of this war for his love!?

The naughty education contest for the protagonist's life begins!!