Sister Princess (シスター・プリンセス)

Sister Princess (シスター・プリンセス Shisutā Purinsesu) is a popular Japanese bishōjo series authored by Sakurako Kimino and illustrated by Naoto Tenhiro. It began as a serialized light novel series in 1999. In 2001, a manga series and a bishōjo game for the Sony PlayStation were released. Sequels to the game were released for the PlayStation and Game Boy Advance. Sister Princess was also adapted into a Japanese anime television series.
The history of Sister Princess can be traced back to March 1999, when it first appeared in the monthly Dengeki G's Magazine. Originally, there were only nine sisters, not twelve. Readers were able to vote for their favorite sister and following the tremendous response from the fans, the magazine decided to serialize Sister Princess.
In March 2000, a decision was made to renew the series, this time by featuring the short letters that the girls wrote to their beloved brother. Three more sisters were added to the mix: Haruka, who came from Germany; Yotsuba, who came from England; and Aria, who came from France.

# Reference
http://en.wikipedia.org/wiki/Sister_Princess
Sister Princess - Complete Collection(Amazon.com)