Based on the erotic game by Pachi Pachi Soft.

After becoming a mathematics teacher for his sister Miu and his friend Riko, Kanata realizes that he feels more popular than he is at school. Kanata is an expert in mathematics, which makes him feel a dilemma when Riko likes him and also behind his tsundere sister who feels jealous because his brother is always looking at other girls breasts than Miu own.

(Source: Nekopoi which has Been Translated)