Futari no Aniyome

Shin'ya's school days are boring until Akane, a woman whom his brother was to marry due to a family arrangement, started attending Shin'ya's school. Shin'ya remembers meeting Akane when they were younger, and that they made a promise to be together when they were older. In order to make Akane his and his alone, he gets his sister-in-law Sayumi to teach him every way possible to please a woman. After he approaches Akane, and they begin to make love, the gradual distance between the two begins to close and affects everyone around them…

[From ANN]