姫三妹～堕ちゆく三姉妹～

The protagonist, Iijima Naoto (changeable), became an orphan after his father died and his older sister went missing. He was then adopted by Sakurai Yuuzou, a prominent businessman and politician, and came to live with Yuuzou's three daughters. But one day, he found a photograph in Yuuzou's study, and then he realized that he had been deceived all this time. The photo showed his older sister being raped by Yuuzou, leaving her in a miserable state. His father, who had been hospitalized in a mental hospital, had told him about Yuuzou's cruelty, but he didn't believe it because he thought it was just a delusion of his mentally ill father. But now the photo and his father's words are linked, Naoto decided to take "revenge" on Yuuzou... [Translated and modified from Gyutto]