The No Kiss Contract (Jensoo)

Based on the book with the same name by Nan Campbell:

Despite being the youngest partner at her firm, corporate attorney Jisoo Kim is gunning for a recently vacant name partnership-it would mean finally having the power to make a real change in her conservative workplace. But eighty-hour workweeks don't allow any time with her much younger half-sister, Cherry, who doesn't seem to need Jisoo in her life anyway.

Jisoo aims to get back into Cherry's good graces by throwing her a big birthday party, but that means dealing with Cherry's other half sister, Jennie Kim. To say Jisoo and Jennie don't get along is putting it mildly.

When a misunderstanding leads to her boss's assumption that she and Jennie are headed to the altar, Jisoo sees a way to leapfrog over the competition straight into one of the top spots at her firm. She just has to convince Jennie to be her faux fiancée in order to do it and keep her growing attraction in check, of course.

(This isn't incest. They are step sisters that share a very young sibling.)