Why do Shantou people in Guangdong call others brothers and sisters when they are obviously older than others?

There are two possibilities. The first possibility is out of politeness, because Shantou people usually like to call others big when they go out, so they often see other people's brothers and sisters. Second, it may be an intergenerational problem. Even if they are older, they are younger than each other, so they call others brothers and sisters! Please adopt it, thank you!