Jinhee feels threatened by Hyunji's advances on her brother, so she tries to claim him by giving him a practical sex ed session. They get cut short because of the AC Repairman.
[NON-CONSECUTIVE] Jinhee tries to get off by masturbating alone in the bathroom but it's not enough. She accidentally goes into her Sangyeo's room thinking it's hers, only to find her brother's rock-hard dick towering above anything. At that moment, boundaries became meaningless, as she let go of reason & started acting according to her instinct, full of repressed lust that has been neglected for very long time. She finally gets off after using her sleeping virgin brother to her heart's content.
Jinhee barges into the shower while her brother was still masturbating. She shamelessly takes off her panties & tells him to hurry out so she can shower, leaving him fantasizing about doing her on the spot.
Cha Sang-Yeo