I'm looking for a novel title.

I've seen it before, but I forgot the title.
The male protagonist fell in love with his sister (someone else left it at his doorstep). The younger sister wanted a sister-in-law, so the male protagonist went to look for the female protagonist (they seemed to have met in a coffee shop). The two of them started dating. Later on, the male protagonist's sister met the man she loved. The male protagonist was very angry. In front of the female protagonist, his sister, and his family, he said that he loved his sister. The female protagonist was just a substitute for her sister. If she wanted to marry her sister, she would die. The male protagonist's mother begged her sister to marry the male protagonist for the sake of the male protagonist. The male protagonist understood that he loved the female protagonist at the wedding. Later, the wedding made fun of him and the male protagonist went to look for the female protagonist.
It's a love story.

Answer 1

This novel was called " The Full-time Expert."