Shumatsukon

Tsukiko, a good-natured young woman, has always lived in the shadows of her elite sister Yoko. Ever since childhood, Yoko was praised and complimented for her talents and appearance. Throughout her life, Tsukiko felt inferior to her sister, and hid her true feelings behind her cheery character.
Meeting Hyo, the love of Tsukiko's life appeared to hold the promise of a bright future for Tsukiko. When Yoko attempts to marry Hyo's older brother Jun, this threatens Tsukiko's marriage to Hyo. Tsukiko realizes that it is time for her to put her own happiness first. Tsukiko makes a desperate plea for Yoko to give up marrying Jun. But Yoko bluntly declines Tsukiko's request, saying that Tsukiko is not suited for Jun and Hyo's family. (Source: Dramawiki)