Why is second sister Zhang Xin called second sister?

Zhang Xinyi plays the second daughter Lin Xia in Beijing Love Story. Zhang Xinyi plays a heartless crazy girl in Beijing Love Story. She is cheerful and loves to laugh. The second woman, Lin Xia, is very attractive, so fans call her Second Sister. Beijing Love Story is a love TV series directed by Chen Sicheng and starring Chen Sicheng and Jerry Lee. 20 12 1.8 premiered on Zhejiang Satellite TV.