Um... why would she be? Who said she is the real daughter of the duke? Noone.
She was adopted by the duke because she has a power(?), and she has to marry the prince. So if she isn't a noble she can't marry him.
And there are many of other ppl who has the same hair color and eye color like you. Maybe the duke adopted him because he resembles to himself.
Or maybe a relative of his.
I see, I got confused for a sec when he got emotional and called her ‘my daughter’ when she went missing during the horse training chapter. The fact that she is the child of the prophecy and will marry the crown prince in the future but has to be a noble to do so makes sense, thank you :) and as for the bro I kinda jumped to conclusions lol, excited to learn the reason for his adoption, maybe he has a special power too
hummm so, is the mc the real daughter of the duke and the blonde hair woman? but they didn't get married... and how come the brother looks like an exact little version of the duke?