correct me if im wrong but arent they siblings or something😭

The grand duke and the prince??
Noo
Grand duke is born from a omega commoner and an alpha grand duke

While prince is born from the bitch and that bastard father

an* omega

I think Seme calls uke hyung(older brother) which is a term used by males to address males older than them in Korea. It's like gege in Chinese. If you've read other korean or chinese Manga's then you'll come across it often.

He is his cousin tho

...
Sorry, I didnt see where it indicate that.
Can you tell me where?
Σ( ￣□￣||)

i’m sensing inbreeding in the future…..

They are not. Rishar is calling Etienne hyung, that is loosely translated as brother here