Need some help with a delicate issue arisen in our weekly game:

Sasha Gunnarson was a Rjurik, who swore loyalty to the Emperor of Anuire (in our campaign Anuire is a unified Empire). He became the Regent of 3 regions of Alamie, renamed Cuirenie.

He had a son, Hrothwulf, and died shortly after. Hrothwulf's mother, also a rjurik woman, died too not long after. Sasha's sister, a priestess of Kirken named Rowena, decided to abandon the Anuirean ways, and travelled to the Giantdowns to take care of little Hrothwulf in the Rjurik way.

She left the rule of Cuirenie to a nearby regent (a trusted adventuring companion of Sasha).

Now, 25 years later, Rowena is dead, and Hrothwulf is carving a realm out of the Giantdowns. He's already unified 7 out of 12 provinces, and he's confident he'll be king in a year or two. He has never returned to Anuire, and has no interest in ruling Cuirenie.

Now the issue: the Emperor of Anuire, seeing how good Hrothwulf is doing, reminded Hrothwulf that, since his father was his vassal, he is too. And consequently, he wants to annex the Giantdowns to his Empire.

Hrothwulf has already stated that he's willing to "give up" the lands of Cuirenie, to be free of the Emperor's claim, but the Emperor has refused. Would the Emperor be right? Is Hrothwulf a vassal, just because his father was?