Secret Book: Turbulent Cloud (1974)

In the early Shōwa era, university student Gōji Murata boards with the Itō family, old friends of his late father. One day, the family's daughter Tomiko catches him reading an erotic book. Rather than being shocked, she’s intrigued, and soon gives herself to him. That night, her older sister Kyōko, having learned what happened, quietly slips into Gōji’s room..."