Were Russian morganauts like the Counts and Countesses Osternburg, Zarnekau, Merenberg, Hohenfelsen etc. styled Сиятельство, just like Russian Counts, even though they were created by German sovereigns like the Grand Duke of Oldenburg, the Prince of Waldeck and Pyrmont and the Prince-Regent of Bavaria? (And thus probably actually technically held the lesser style of
Hochgeboren, высокородие, like non-mediatized German counts.)
And of course there is this question, which recently popped up in Mandie's fan fiction story "Lizya & Georgie ~ Love, Power and Tragedy" in the Having Fun section:
NB the dramatis personæ are a young Russian grand duchess and a young Bavarian prince:
“Sie sprechen Deutsch, Herzogin (You speak German, Duchess)?” Georgie informally asked in his native tongue. (And a very stupid question too! - Tom added).
I understand that the "Duchess" in question is a Russian Grand Duchess? In that case she is
Großfürstin (Grand Princess) in German. I think that to a certain extent social equals can adress each other with titles in German as in Russian, but in German I have a feeling this is restricted to lower titles like
Gräfin, Countess, i.e. the ones where the titles usually is preferred to the more uncommon predicate. But I think it would be more usual to add, à la mode française, a
Frau, or in the case of a young girl as here,
Fräulein: "
Sie sprechen Deutsch, Fräulein Großfürstin?" But I am not at all sure here - it is indeed a very interesting question: How did royals of equal age who didn't know each other intimately enough to use those well-known nicknames Ducky, Sandro, Vicky, Toria, Bertie, Nicky, Minny etc. really adress each other?