A secondary character in my historical fiction novel is a Russian concert pianist who married into a 19th century noble family. For various reasons I would very much like to make her the daughter of Grand Duke Vladimir Alexandrovich who was a son of Emperor Alexander II of Russia.
In real life the Grand Duke had only one daughter. Is it "permissable", for purposes of my novel, to create a second daughter? Perhaps even a daughter who was exiled from the family for some reason?