First of all, the official history is not recorded. Luo Guanzhong's Romance of the Three Kingdoms is written according to the overall framework of Chen Shou's The History of the Three Kingdoms. However, in the History of the Three Kingdoms and Historical Records, Liu, Guan and Zhang did not mention becoming brothers. There is no record of these three people becoming sworn brothers in history, but if they are really sworn brothers. After Guan Yu was killed, Cao Pi asked Minister Liu Bei whether he would send troops to attack Wu and avenge Guan Yu. Why didn't Liu Bei mention their brother's name at all?
Second, different identities. We often say that the door is in front of us. In fact, this is an ancient concept of door. Family system, also known as family system, is a special hierarchical system in China feudal society. The patriarchal clan system was formed in the Eastern Han Dynasty and prevailed in Wei, Jin, Southern and Northern Dynasties. Liu Bei lived in an era with strong family values. Before Liu Bei became rich, although he was only a peddler selling straw sandals, his background was very noble.
Liu Bei often thinks that he is the great-grandson of Emperor Wu of Song and Empress Zhongshan Jing. Liu Bei was born in poverty, but his teacher was Lu Zhi, the magistrate of Jiujiang County. His classmates all come from such a rich and powerful family as Gongsun Zan. Under the historical background at that time, Liu Bei, who once wanted to achieve great things, also attached great importance to the concept of family status. Unlike Liu Bei, Guan Yu was a fugitive after killing people. Although Zhang Fei has a little property, he is only a civilian. These two men are not on the same level as Liu Bei.
Third, and finally, the issue of age. According to relevant historical research, Guan Yu is one to two years older than Liu Bei. Zhang Fei is indeed a few years younger than Liu Bei. Even if these three people really swore an oath, the eldest brother should be Guan Yu, Liu Bei can only be the second brother, and Zhang Fei ranks third, which obviously contradicts the Romance of the Three Kingdoms.