A Secret Lesson With My Younger Sister summary: “I heard you’re really good at sex.” Jinyoung, known among his friends as a sex master, is actually inexperienced and as single as could be.However, when he’s questioned by his 20-year-old sister, Bomi, Jinyoung pretends to be experienced and lies. After that day… Every night, my sister now comes to my room without our parents knowing. This is a problem.