Nee-san no Kareshi ~Akogare no Nee-san ga Shin'yuu to Tsukiai Hajimemashita~

The season is early summer. The protagonist is living an ordinary but enjoyable school life with his proud older sister Himekawa Iori and his best friend since childhood Tagami Seiji. The protagonist is very conscious of his sister as the ideal version of the opposite sex, but at the same time, he almost gives up on the idea as 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 illicit feelings for 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. One day, however, as the summer holidays approach, their relationship changes. When Iori visits the protagonist's room, she witnesses him masturbating to her. She wonders if there is anything she can do to make him give up his feelings for her.

After much struggle, the answer she comes up with is to talk to Seiji about it, who has a lot of experience with male-female relationships. Seiji, who has wanted to make Iori his own woman since childhood, pretends to be her advisor and tries to fulfil his own desires. After persuading Iori that it is for her brother's future, Seiji asks her to go out with him for a month. 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 sight of his sister gradually being transformed to Seiji's liking. As if to spite the protagonist, Seiji's 'lover lessons' for Iori begin to escalate.

[Translated from Getchu]