SM no Susume -The Seeker ZERO-

Satsuki finally confesses to her big brother Taiki that she is in love with him. However, Taiki is a full-fledged sadist! Notwithstanding, Satsuki refuses to let go of her feelings for her brother; with some confusion, she accepts his inclinations.

At about that time, Taiki's junior from high school, Kaoru Sakuraba, and Satsuki's senior, Taeka Yukimoto come to stay with them. All three these girls are infatuated with Taiki. They all offer to begin training to become his girl-slaves. And so, the training in their new profession begins...

[From gamelexgs]