Why do people say Oh brother instead of Oh sister?

The phrase "Oh brother" is often used as an expression of frustration or exasperation, and it has historical roots in the way language and familial terms have evolved. Traditionally, "brother" has been used more broadly in English to convey a sense of camaraderie or shared experience, which may make it feel more fitting in certain contexts. Additionally, cultural references, such as literature and media, have popularized the phrase, leading it to become more commonly used than "Oh sister." Ultimately, language usage can be influenced by social norms and conventions that favor certain expressions over others.