Where to Watch My Friend's Older Sister

Joon-soo learns that his friend Hee-gyu's sister Hee-jin has no where to go after being scammed, so he suggests living with him. The two start living together. They are close like brother and sister but they can never be that. The two start getting funny and Joon-soo can't go to sleep at the thought of Hee-jin sleeping just a wall away from him. Mi-na, Joon-soo's girlfriend notices Joon-soo changing and seduces Joon-soo at his house where Hee-jin is. Is it the girlfriend or the sister?