Pink no Curtain

After breaking off her engagement, Noriko Okuyama moves in with her brother Satoru. Noriko is having an affair with Soichiro Mitamura, a screenwriter, and she is so captivated by his sexual prowess that she often masturbates while thinking of him. Her brother Satoru feels sorry for her situation, but her uninhibited behavior causes taboo desires to grow within him. (Source: Mangapedia, translated)