An Affair: My Sister-in-law's Love(2018)

Original title: 정사 : 처제의 사랑

Eun-joo is a twenty-year-old pre-college student who lives with her sister Eun-hee. Eun-hee is worried about her sister's academics and hires Je-hyuk as her tutor. Eun-joo falls for Je-hyuk as time goes by. They end up making love. A few months later, Eun-joo improves her grades and passes her tests. However, Eun-hee announces that she's getting married to Je-hyuk and Eun-joo doesn't want to lose him...