It was obvious to Chu Yun that there was a lot of bad blood between Xiao Zai and the First Prince, who, unlike Xiao Zai, resembled his father in everything. Including the arrogant cast of his eyes, and the haughty tilt of his chin.
It was clear from his demeanour that the First Prince never once questioned his claim to the throne.
He was brought up with the clear certainty that he would be his father's heir -- which was curious considering he wasn't the child of the late Queen Consort. From what Hua Nanyi had told him, the Queen had only one child, an alpha girl, all her other pregnancies resulted in miscarriage. Except her last one, which she managed to carry to term.
Only for both her and the child to die during delivery.
In Xin, the first alpha child of the official spouse inherited the title. Regardless of primary gender.
Apparently, that wasn't the case in Zui.