@GuildMasterJin
In many ancient states, children who were of interest (read: noble, but who knows how magic counts) that did not have guardians would be assigned one without their consent by the monarch or ruler. In fact people often paid quite a bit of money to the monarch for this honor because it allowed them to make a strategic alliance with the child into adulthood, run that child's estate for profit, and make advantageous marriage alliances that would benefit themselves on their behalf.
The count could also just say she's a commoner foundling child who appeared in (his domain?) and as the a high administrator he has a right to take on and decide her fate legally. It's not like feudal societies were big on childrens rights or conflicts of interest, and everyone clearly sees her as a child.