Choukyou Daikounin Another Story ~Uragiri no Daishou~

Mia has a boyfriend Sail. He comes over for passionate dates, but one day he is caught in bed with sister Sophia. Soon Sail and Sophia start living together in a new place. Mia is furious with Sophia. One day Mia overhears a rumor that there is mansion where women are bought and trained into sex slaves allowing seller to observe the process. Mia sells her sister to the mansion owner, and training begins.

[From vndbreview]