Why brother Wu is brother Wu?

I don't know martial arts, but in ancient families, men and women were counted separately. The boss is the boss, and the second is the second. The eldest daughter is the elder sister, and the second daughter is the second sister. Men and women are called by age. Yongqi has four brothers, even if they don't all survive, he will rank fifth among men, that is, the fifth brother. Wei Zi is younger than him, so naturally he is called Brother Five. As for the Wudang Seven Heroes, even if there are women, they are Mo Seven Heroes, because they should be arranged according to the entry time.