Urania is an English Princess. To date, she has only been in the main comic, in the context of being rescued by Trelawney Thorpe from a kidnapping attempt which carried her outside of English airspace. She has appeared in person in the (semi-canon) Trelawney side-story The Crown of the Sleeping King, again in the role of a kidnapping victim, who selflessly (if somewhat arrogantly) orders Ms. Thorpe to deal with the villain of the moment instead of rescuing her.
She was also included way back in the Girl Genius Complete List of Absolutely Everybody! where it is noted she is indeed in the (rather ) official line of succession behind Queen Albia. However, hanging around in said line can be "frustrating", as Albia is thousands of years old. Urania is also described as being "popular" and possibly "fast-living".
Since Albia declares that Agatha is to be treated as a Princess of the Realm, it is possible that Albia and Urania are not biologically related.
Possibly Useful Outside Information
In Greek Mythology, Urania is the Muse of Astronomy.