APAK-100 I Ended Up Seeing My Sister (A High-Schooler) Bringing A Middle-aged Man Into Her Room And Indulging In Perverted Sex...Akane Kuramochi

Unbeknownst to her brother, her younger sister, Akane, has grown into an adult, learned nasty things, and brought a middle-aged man older than her brother home. Assuming that her brother is absent, she commits lewd acts with a middle-aged man in her room. My brother, who pretended to be absent, peeped into his sister's room and erected when he saw sexual intercourse with a middle-aged man. What is the action taken by my sister, Akane, on the condition that her brother, who was peeking, shut her mouth?