お姉ちゃんのあまエロレッスン ～世話焼き巨乳お姉ちゃん先生にたっぷり甘えるめちゃエロすくーるライフ～

The protagonist, Yuuya Asahina, lost his parents when he was a child and was taken in by his grandparents far away. He also had to say goodbye to his childhood friends, who were like older sisters to him. What gave him strength was a promise: "When I grow up, I'll be your wife, Yuu-kun." Time passed, and he returned to the town where he used to live to go to a school there. He felt a sense of familiarity towards the school nurse Enomoto Chisato and his homeroom teacher Shiina Mizuki, whom he met at the school. He gradually deepened his friendship with them, wondering if they could be his childhood friends...

[Translated from Official Site]