'Either the Russians addressed one another by titles - or the classic writers (or their translators) had it wrong. I have an acquaintance who is the member of a royal house. While she would never say so directly to a person, if anyone calls her "Princess" I am given to understand that this is a name best reserved for a spoiled cat. It is quite acceptable to call her by her first name when not in public, but where others can hear, it's "Your -- Highness". Period.'
In British usage a princess would never be addressed as 'Princess', though she would be referred to as 'Princess X' or 'the Princess'. Formally, it is 'Your Royal Highness' and, less formally, 'Ma'am.'
As far as Russian usage is concerned, I had better set my cards on the table. I am writing a novel set in 1916-17 of which the hero is a prince of ancient Rurikkid family. I have worked so far on the basis that those outside the immediate family circle call him 'Alexander Alexandrovich' and those rather more distant but still aristocratic whom he meets in society 'Prince Alexander Alexandrovich' or simply 'Prince'. He will also be referred to on occasions as 'Prince Surname'. He is a Captain in the Imperial Guard so, as I understand it, called 'your High Nobility' by subordinates in the military context. The servants will presumably refer to him as 'Prince Alexander Alexandrovich' but how will they address him?
The next question concerns married ladies, specifically widows. Is his widowed aunt 'Princess Vassili Surname' or 'Princess Maria Surname-a'? Is there a difference in usage between widows of princes and those whose husbands are still living? In traditional (and now old-fashioned) British usage, either a wife or widow is Mrs etc Husband's Name unless she has a more elevated title in her own right, hence Mrs Patrick Campbell, Lady Randolph Churchill, Princess Michael of Kent (the late Diana Princess of Wales was never 'Princess Diana'). However, an earl's daughter of my acquaintance who married a naval commander named Page is Lady Cecilia Page. Strictly speaking 'Mrs Christian Name Surname' is only owed by divorcees.
Thank you in advance for advice on this.