After having sex in the empty classroom during lunch time, the siblings came back to the lecture hall & Taesung started fingering his sister during lecture. He made her so horny that she had them hurry back home to fuck where Haeun grants her brother his wish of fucking her backdoor for the 1st time in her life.
Ga-yung pounces on Kyung-Ho at the bar's bathroom but they get interrupted. Once they're out of there, she invites him for a fancy jacuzzi hotel room where they obviously bang to their hearts content.