Ok so basically the current Emperor now is a commoner?

I think he was a son of a duke? cuz her sister is referred to as duchess

But I thought tht he bring his family to bcome the noble. He wants to conquer the imperial authority and get rid of the original imperial bloodline

That is because he wants to revenge the imperial family who killed his lover and destroyed her family. So he wants to completely annihilate the imperial family by making his son who is not of imperial blood, the next emperor. This would be the greatest revenge.

Ohh now I get it