The grandfather of  Duke Peter Alexandrovich was the Duke of what European dynasty of North German Origin
Duke Peter Georgievich of Oldenburg: Duke Konstantin Friedrich Peter Georgievich of Oldenburg (26 August 1812 in Yaroslavl, Russian Empire – 14 May 1881 in St. Petersburg, Russian Empire) was a Duke of the House of Oldenburg. House of Oldenburg: The House of Oldenburg is a European dynasty of North German origin.
i don't know