Kairaku Ochisaserareru Gimai Minami

The protagonist, Norikawa Kouta, lost his parents when he was young, so he was raised by his relatives alongside his cousin, Norikawa Minami. They grew up together like siblings, but as they became older, Kouta began to develop special feelings for Minami. He was aware that Minami thought of him as more than just a brother as well. However, he knew that his adoptive parents and the people around him would not accept their relationship, and he also thought that he would not be able to make Minami happy. And because of that, when it was time for him to enter university, he left home without telling Minami where he attended, deciding to cut off the feelings for his adoptive sister.

One year later, in the room of Kouta, who now lives in the city, Minami suddenly appears. She moves to the city in the spring in order to attend her university as well, she learns of Kouta's address and comes to visit him.

Since then, Minami visits him regularly, treating him the same way like when they were at their parents' house. However, Kouta's feelings that he had sealed away are about to be revived, so he desperately suppresses them.

One night, staying in his room, Minami approaches him and confesses her feelings. But Kouta rejects her, squeezing out the words that he cannot accept her feelings. Then, he encourages her to find someone that is better than him.

Afterwards, Minami no longer comes to his room. Some time later, he sees Minami again, with another man by her side. While Kouta still has feelings for her, as her brother, he can only wish for her to be happy.

However, Minami has already, because of that man, her body is being trained for the pleasures of sex――

[Translated and edited from Getchu]