Shouki Letitia ~Koyoi, Oujo wa Haru o Uru~

Estor Kingdom is blessed with rich nature. It exists in stability for over a hundred years until king falls ill. At this time minister's coup breaks out. As a result, king is imprisoned and the princess and queen are captured. But minister can't reign himself as people would overthrow the usurper. He blackmails princess and queen with king's life and coerces them into being trained by protagonist who is a high class courtesan trainer. They can only leave this building as complete whores.

[From vndbreview]