Question: When Violet Wegner became HRH Princess Pierre of Montenegro, who was her sister-in-law?

Context paragraphs:
Violet Wegner: Violet Emily Wegner, (1887-1960), was the daughter of an Extradition Department detective of Scotland Yard, William T. Wegner, and Arabella Eliza née Darby, who resided in a district of London called Tulse Hill.  Violet made a career in the music halls from the age of 15.  She became known as the "Idol of Berlin".  In 1912, Violet married her first husband, Sergio Francesco Enrico Maria Brunetta d'Usseaux (born 3 March 1885, Torino, Italy), in London.  Sergio's parents were Comte Eugenio Brunetta d'Usseaux and the Russian Countess Zeiffart.  Eugenio was Secretary General of the Olympic Committee that administered the 1908 London Olympics.  Eugenio died in 1919 in mysterious circumstances.  The fate of his son, and Violet's husband, is also unknown although it is suggested he may have been the son Eugenio was trying to reach following the October Revolution.  Whilst traveling in Italy during 1918, Violet met the youngest son and royal prince, Pierre Petrović-Njegoš, of the exiled King Nicholas I of Montenegro.  After some delay, it is said, at the behest of Violet's mother who advised them to await the award of monies claimed by the prince from the new Yugoslav government for the confiscation of his family's property in Montenegro, Violet married Prince Pierre (Peter) of Montenegro in Paris in the April of 1924.  Violet then became HRH Princess Pierre of Montenegro and thus sister-in-law to Queen Elena of Italy as well as a host of other European princes and kings via her husband's siblings.

Phindiwe Sangweni: Phindiwe Dlamini-Sangweni (born 2 December 1963, as HRH Princess Phindiwe Rita Dlamini) is a South African attorney and businesswoman.  She is currently a member of the African National Congress and is CEO of Siyakhuluma Consultancy, a private company.  Phindiwe is a Principal Princess of the Nhlangwini royal family and is a cousin of Mswati III, King of Swaziland.  Her Royal Highness is worth an estimated $30 million from her various investments and shareholdings on the New York Stock Exchange and Johannesburg Stock Exchange.

Princess Dana Firas: HRH Princess Dana Firas ( Dana Nabil; "née" Toukan; born 27 October 1970) is the wife of HRH Prince Firas bin Raad.  She serves as the President of the Board of Directors of Petra National Trust and is a liaison with the The Royal Hashemite Court and Jordanian government ministries.  She was named a UNESCO Goodwill ambassador in June 2017 by the UNESCO Director-General Irina Bokova.  Her advocacy for the cause of cultural heritage preservation and awareness resulted in their inclusion in two World Tourism Organization declarations.

Oscar Wegner: Oscar Wegner is a tennis coach and pre-open-era tour player, author, and creator of Modern Tennis Methodology, a tennis-teaching system which he began developing in 1968.  He is Founder and President of Oscar Wegner Enterprises, Inc. and Tennis Kids For Life, Inc. a non-profit corporation.  Other projects he has created include The Modern Tennis Methodology Coaches Association, The Oscar Wegner Elite Training Program and The Oscar Wegner Modern Tennis College.  He teaches publicly and privately throughout the United States and internationally, contributes to tennis publications and events, and publishes TennisTeacher.com.  He is a USPTA Professional 1, the highest tested rating possible in that organization.  Wegner became an American citizen in 2005.  He currently resides and coaches in Clearwater, FL .

Mount Helena (British Columbia): Mount Helena is a mountain located at the Queen Reach arm of Jervis Inlet and behind Princess Louisa Inlet.  Mount Helena is part of the Pacific Ranges of the Coast Mountains in British Columbia Canada.  The mountain was named during the 1860 survey by HMS "Plumper" who charted all of the known area and named the mountain after HRH Princess Helena Augusta Victoria “Lenchen” who was the fifth child of Queen Victoria and Prince Albert of England.

Hteiktin Ma Lat: HRH Princess Hteiktin Ma Lat, also Tin Tin Ma Lat (Burmese: ထိပ်တင်မလတ် ; 13 October 1894 – 1965), is a Princess of Burma and one of the senior member of the Royal House of Konbaung.

Elena of Montenegro: Princess Elena of Montenegro or more commonly known as Queen Elena of Italy (8 January 1871 – 28 November 1952) nicknamed "Jela" was the daughter of King Nicholas I of Montenegro and his wife, Milena Vukotić.  As wife of Victor Emmanuel III of Italy, she was Queen of Italy from 1900 until 1946 and Queen consort of the Albanians from 1939 until 1943.

Basmah bint Saud Al Saud: HRH Princess Basmah bint Saud bin Abdulaziz Al Saud (1 March 1964 in Riyadh, Saudi Arabia.)  is a Saudi businesswoman and a member of House of Saud.  Since 2010, she has lived in Acton, London, In 1976, due to the outbreak of civil war in Lebanon, HRH moved between London and America with her mother.

Undercover Princesses: Undercover Princesses is a BBC Three reality TV show which took three royal claimants from foreign cultures and placed them in Ingatestone where they had to 'live and date' like normal people.  The idea for the programme came from the 1988 Eddie Murphy film "Coming to America".  The contestants are Xenia Gabriela Florence Sophie Iris, (Xenia Prinzessin von Sachsen) Princess of Saxony and Duchess to Saxony, Germany, b. 1986 (great-great-granddaughter of King Frederick Augustus III of Saxony); HRH Princess (Omumbejja) Sheillah Cinderella Nvannungi of Buganda, Uganda, b. 1982 (daughter of Prince (Omulangira) George William Juuko Walugembe Kassabbanda) although this claim by her is disputed; and HH Princess (Nawabzadi) Aaliya Sultana Babi of Balasinor, India, b. 1974 (daughter of Babi Shri Muhammed Salabat Khanji II, Nawab of Balasinor).

United Kingdom Antarctic Heritage Trust: The United Kingdom Antarctic Heritage Trust (UKAHT) is a British charity, registered in 1993 and re-registered in 2015.  The UKAHT is a member of the Antarctic Heritage Trust coalition.  The UKAHT's Patron is HRH Princess Anne, The Princess Royal.

Supporting paragraphs: Violet Wegner Elena of Montenegro
Answer: Elena of Montenegro