My Friends Older Sister

Joonsoo learns that his friend Heegyus sister Heejin 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 Joonsoo cant go to sleep at the thought of Heejin sleeping just a wall away from him Mina Joonsoos girlfriend notices Joonsoo changing and seduces Joonsoo at his house where Heejin is Is it the girlfriend or the sister