Well yoonshin' s sister wanted him to work under dogyuk firm and that too specifically under sehun. Sehun at 1st was skeptical about him joining under as subordinate. After sometime he did get his assurance that yoonshin is Indeed a good a lawyer and kept him in the work.
Now ikyung (sister) is actually getting physically and mentally abused by her husband . So she is preparing for a divorce law suit. She is gathering all sorts of evidence that she will use later. And she states her plan to yoonshin not long ago in chapter..while sehun saw them from afar discussing something. Yooshin actually think it would be great if sehun agreed to take this case but he knows he doesn't deals with this type of case he would rather be on company side( ikyung husband is a director of a big companie). For now they are a bit lovey dovey ig. But later ikyung 's case will be a major part of the story.
I tried to summarize to my best English is not my 1st language
Yep. Ikyoung is basically trying to use Sehun as a wall to protect Yoonshin when her divorce goes through. And Sehun, having inhuman instincts, deduces that he is being used by Yoonshin (though Yoonshin has no clue about Ikyoung's plan when he falls for Sehun) then they sort stuff out.
Please help me remember the storyline