Natural 2: Duo
Japanese title: ナチュラル2 デュオ
Type: OVA
Episodes: 4
Source: Visual novel
Genres: Hentai
Studios: Arms
Producers: Green Bunny
Duration: 28 min per ep
Rating: Rx - Hentai

Chisato and Kuu lost their parents years ago and were taken in by an Old man and his grandson, Chisato and Kuu became good friends with his grandson however he left and never came back. Now. Years later Chisato met up with her "Onii-chan" her lost love.

(Source: ANN)