Why is the eldest son Yang called?
First of all, the Yang family has seven brave children, but not the sixth. On the contrary, Yang is the oldest child in the Yang family. Yang showed absolute military talent from an early age. When he grew up, he even won the battle with Yang Jiajun. It can be said that Yang was born to fight.
In ancient times, people believed in astronomy. The ancients called Sirius the six-lang star, hence the name Yang Liulang, a soldier. In fact, Yang also deserves the title. Thus, Yang Liulang is called Yang Liulang because he is brave and good at fighting, which is a title, not because he ranks sixth in the family.
Yang hardly failed in the battle with the Liao army. In the eyes of Liao army, Yang is like a Sirius, so Yang deserves the name of Sirius, so he is called Sirius. Yang Liulang is a compliment, not an ordinary title. I hope that after reading my answer, you can correct your misunderstanding about Yang Liulang.