Why did Gao Qiu appear as the second brother of senior high school?

Gao Qiu used to do odd jobs in a drugstore. During the Song and Yuan Dynasties, he called the shopkeeper the eldest brother and the clerk the second brother. Dong Jieyuan's "The West Chamber" Volume 1: "Qinghe Junrui, Emperor Dian has the right to live at any time, and beautiful people are close friends. When he wants to be distracted, he has nowhere to go. He was wondering when the second brother pushed the door in. " Second, in the north, calling people second brother means being close. I remember reading it in a book. It seems that Shandong is taken as an example.