He's not
ok thank you
╮(￣▽￣)╭
It is true they haven't given a character review as to relationships other than the MC's mother is the kings sister, but I think I'm taking it for granted that all of the historical stories have Dukes related to the king, like brothers, or uncles, and such. "You can't be King, but you can be one step away/next in line" deal. The family could go back hundreds of years, and not be that close time wise. Of course this is all my imagination and has no bearing on any story ever written!
I think they are duchy because their family used to be the royal family of their land before they become part of the empire
how is mikahil related to the king and queen again