Who married Prince John Konstantinovich of Russia?

Prince John Konstantinovich of Russia, formerly Grand Duke John Konstantinovich of Russia (Russian: Иоанн Константинович; 5 July 1886 – 18 July 1918), sometimes also known as Prince Ivan, Prince Ioann or Prince Johan, was the eldest son of Grand Duke Konstantin Konstantinovich of Russia and Yelizaveta Mavrikievna, née Princess Elisabeth of Saxe-Altenburg. He was described by contemporaries as a gentle, religious person, nicknamed "Ioannchik" by his relatives.

Princess Helen of Serbia (4 November [O.S. 23 October] 1884 – 16 October 1962) was a Serbian princess. She was the daughter of King Peter I of Serbia and his wife, the former Princess Ljubica of Montenegro. She was the elder sister of George, Crown Prince of Serbia and King Alexander I of Yugoslavia. Helen was also a niece of Queen Elena of Italy, Princess Anastasia of Montenegro (or "Stana"), wife of Grand Duke Nicholas Nikolaevich of Russia and of Princess Milica of Montenegro, wife of Grand Duke Peter Nikolaevich of Russia, the women who introduced Grigori Rasputin to Tsarina Alexandra.