A schoolgirl is about to confess to the guy she likes, only to find out he's banging his stepsister because of a condition that makes him a total sexual beast. The stepsister then "trains" the new girl to satisfy him.
It's pretty long iirc. Image more or less unrelated.
bump for interest
Continue to train your Google-fu, padawan: the gap is strong with this one. Takuji, oreimo