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. Duke Peter Georgievich of Oldenburg:  He was the grandfather of Duke Peter Alexandrovich of Oldenburg as well as grandfather of Grand Duke Nicholas Nikolaevich, General of the Imperial Russian Army during World War I. House of Oldenburg: The House of Oldenburg is a European dynasty of North German origin.
House of Oldenburg