The Kim and the Kwon

Kwon Yuri and Kim Taeyeon are cousin.

They grow up together and Taeyeon treat Yuri as her own sister and she will do anything for her since she only has her as her family after her parents died in a car accident.

Yuri on the other hand is kinda wild and she's a player and she further her study in a university at the United State.

The Jung

Jessica Jung and Tiffany Jung are twins. Tiffany was left alone in America because she attend the local university there where as Jessica needs to help her father run their business in Korea so the two sister got separated.