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. 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. His great-great grandson, Nicholas Romanov, was the President of the Romanov Family Association until his death in 2014.
Grand Duchess Xenia Alexandrovna of Russia Grand Duchess Xenia Alexandrovna of Russia (Russian: Ксения Александровна Романова ; 6 April [O.S. 25 March] 1875 – 20 April 1960) was the elder daughter and fourth child of Emperor Alexander III of Russia and Empress Maria Feodorovna of Russia (née Princess Dagmar of Denmark) and the sister of Emperor Nicholas II. She married a cousin, Grand Duke Alexander Mikhailovich of Russia, with whom she had seven children. She was the mother-in-law of Felix Yusupov and a cousin of Grand Duke Dmitri Pavlovich of Russia who, together, killed Grigori Rasputin, holy healer to her nephew, the haemophiliac Tsarevich Alexei Nikolaevich of Russia. During her brother's reign she recorded in her diary and letters increasing concern about his rule. After the fall of the monarchy in February 1917 she fled Russia, eventually settling in the United Kingdom.