The story of a younger brother who falls in love with his future sister-in-law... without knowing that her fiancé is gay.
Yes, the plot seems interesting although not innovative.
However, while the first half is enjoyable and I liked it a lot, the second half is a letdown. It feels like the screenwriters lost themselves and resorted to gaudy drama.
On the other hand, I must say that the leads relationship is cute and adorable to see (although it was painful to watch the final eps filled with overdone drama). Also Jeon Jong-Seo acting is superb, I'm expecting a lot of her future roles as I became a big fan! Moon Sang-Min shined a lot as well and since he's still pretty young we'll probably see a lot of him in the future, mainly with ML roles.
Back to the plot, they wasted a lot of potential by not exploring Do-Han, the gay brother. Almost all the time the character is put on standby, unconscious about what happens despite his life being on the line. My biggest disappointment is that his development is literally non-existent – ​​he enters into a fake marriage with his best friend to hide his sexual orientation from his family, and as the story progresses there is no moment where he understands the burden he has placed on Ah-Jung, as well as that coming out of the closet is the only way to have peace with himself.
There are many other flaws consisting of not exploring a character and their drama, while a lot of screentime is wasted with Ji-Han and Ah-Jung denying their feelings instead of accepting and fighting for their happiness together.

TL;DR

If you don't mind overdone drama, go ahead and watch it, but do know that it fails by not developing some characters.
If you do mind, it's still worth watching, especially because of Jeon Jong-Seo – I hope you fall in love with her acting skills as I did. However, prepare yourself to see quality dropping in the second half.