Imouto ga Kanojo ja Dame desu ka

In the midst of a family reunion, the protagonist is shocked to discover that his younger sister has become involved with his former girlfriend, who he had thought was long out of the picture. The reunion serves as a catalyst for the protagonist to reevaluate his feelings and relationships, particularly his complicated past with this former girlfriend. As he navigates the complex web of emotions and loyalties, he must confront the truth about his own desires and the consequences of his past actions.