Ochi Yuku Seijo

Rock Raven used to be a student at a religious school, but he is now a servant for a rich merchant. One day, this merchant asks him to kidnap Sister Marleese from an abbey. Rock Raven sneaks into the abbey, as "Gray," a priest who has lost his religious beliefs. Marleese is the illegitimate child of a nobleman who recently died, and only a few people in the abbey know about it. The merchant gets the information from someone and tries to get the nobleman's inheritance by becoming Marleese's financial guardian.

Even priests cannot easily contact sisters in the strict abbey. In the beginning, players may only contact a teacher named Alexis. You'll start with her in your "sister raping." Next, you'll go after Rio, Merleese's guard, and Christabel, the abbey supervisor. You can even make Merleese your sex slave!

[From gamelexgs]