姉さんのカレシ ～憧れの姉さんが親友と付き合い始めました～

The season is early summer. The protagonist is living an ordinary but enjoyable school life with his proud older sister, "Iori Himekawa", and his best friend from childhood, "Seiji Tagami". The protagonist is very conscious of his sister as the ideal opposite sex, but at the same time, he almost gives up on the idea that she is unreachable. The older sister, Iori, has feelings for her younger brother, but is reluctant to reveal them outwardly because she fears for his future. His best friend, Seiji, who was born the only son of a wealthy family but grew up in a distorted family environment, is starved for affection and harbors ill feelings towards Iori. The three of them, each with their own mental distortions, are still on good terms on the surface and continue to have a good relationship. However, one day near the end of summer vacation, a change in the relationship between the three comes. When Iori goes to visit the main character's room, she happens to see him masturbating with her as his bait. She wonders if there is anything she can do to make him give up his feelings for her.
The answer she comes up with is to talk to Seiji about it, who has a lot of experience with men and women. This is the first time in her life that she's been able to get her hands on a girl. "He convinces Iori that it is for the sake of her brother's future, and gets her to agree to a one-month relationship with him. Thus, Iori and Seiji begin their relationship as lovers. As Iori's younger brother and Seiji's best friend, the protagonist tries to support their relationship, but...
He spends his days in agony at the appearance of his older sister, who is gradually dyed to his liking. As if to punish the protagonist, Seiji's "lover lessons" for Iori gradually escalate.

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