The grandfather of  Duke Peter Alexandrovich was the Duke of what European dynasty of North German Origin
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.
i don't know