Spoiler please!

I read some spoilers that he is not MC's father and Vienna is not really his "niece" but his natural daughter.
However, since MC inherits the title from her mother it doesn't matter. He only got title and power temporarily till MC becomes of age. Thats why he kept forcing MC to "acknowledge" Vienna instead of doing it himself, since if MC acknowledges Vienna as of her family, Vienna gets both family name and status.