Shintaisou: Shin
English name: Princess 69: Midnight Gymnastics
Genres: Hentai
Type: OVA, Episodes: 2
Aired: Jun 24, 2005 to Sep 30, 2005
Studio: AT-2
Source: Visual novel
Rating: Rx - Hentai
Synopsis:
There's a secret competition called "dark rhythmic gymnastics". The entrees complete their beauty and lewd by their technique more difficult than rhythmic gymnastics. 

Haneoka Fubuki is a little brother-in-low of Haneoka Miku who's been missing. While he is looking for his sister, he finds her at the house of Tomomi, Miku's classmate. Acceding to Tomomi, she is trapped in the dark side of "dark rhythmic gymnastics". In order to help her out, he resolves his mind to become a coach of "dark rhythmic gymnastics", and he gets close to Fujimiya Koyuki who was chosen by Tomomi not knowing his fate... 

(Source: AnimeNfo)