The peers in Vietnamese families are not called according to their age, except their siblings, who are called according to the size of their predecessors. If he is behind in the previous generation, his children, no matter how old, call the children of the people in front of him brothers and sisters. In other words, as long as your father is the biggest among his peers, no matter how young you are, other peers still call you brother. This is just like the habit of taking the eldest son as the heir to the throne in the complicated interpersonal relationship of the royal family in China. Other parents and children, as neighbors, with the same ceremony.