Kokone was studying in her room one day when her twin sister, Izuru, returned home. Izuru immediately went to their elder brother's room and the two began to make love to each other. Being twins, Kokone and Izuru are able to share feelings of pain and pleasure. Thus, unable to resist the waves of pleasure, Kokone began fingering herself until both rooms simultaneously echoed with the twins' cries of orgasm.
Annoyed at them, Kokone decides to steal their brother from Izuru.
(Source: AniDB, translated and adapted from official site by Cranston)