My Little Sister's Friend (2018)

Min-soo is at home since his discharge from the military. His sister Soo-min and her friend So-ri come home. Min-soo is surprised to see the 'little girl' he once knew had grown up to be such a mature young lady. One day, Min-soo is watching porn when he runs into So-ri. He freezes, but So-ri seduces him, "Want me to satisfy you?"