Selen

Two sisters, Kaho and Kasumi, are eyes deep in debt thanks to their now deceased father. Ookura Hiroyuki, a student of Kasumi who has a huge crush on her, finds out about her financial difficulties and offers to help out... but, only if... ~ from anidb

A long time ago, the Rindo Clan was cursed by a vengeful spirit so that no male children would ever be born. To counter the curse, every three generations, the Rindo must beget children with a family possessing strong spiritual energy. Ever since that day, the Higetsu family have been important and well-treated servants of the Rindo. It is their duty to mix their bloodlines every three generations with that of the Rindo heir. Time passed and the practice continues to this day. The current heir is Naoto whose fiancee, Suzune, passed away in an accident. Now, he is required to find a replacement.

OAV Series. 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.