Kyouhaku Yuutousei Shimai ~Furyou Group Shuudan Tanetsuke Ochi~

Aoi Masaki, the upright student council president --
Her little sister, Kaede Masaki --
Both have caught the eye of a gang of delinquents.
Aoi is lured into their trap, and her violation is recorded on film.
Using the video as blackmail, the group forces Aoi to tend to their every desire.
Worried about her sister, Kaede follows her one day, and is shocked to find her being
violated by the gang. Unfortunately for her, the delinquents hear her gasp of surprise,
and now the two girls will never be the same...

From DLsite English