Question: When was Archduke Elisabeth Marie of Austria's mother born?

Context paragraphs:
Princess Stephanie of Windisch-Graetz: Princess Stéphanie of Windisch-Graetz (April 4, 1909 in Ploskovice, Bohemia – May 29, 2005 in Uccle, Belgium) was the daughter of Prince Otto Weriand of Windisch-Graetz (1873–1952) and Archduchess Elisabeth Marie of Austria (1883–1963), only child of Crown Prince Rudolf of Austria and Princess Stéphanie of Belgium.  She was the great-grandchild of Emperor Franz-Joseph and Empress Elisabeth ('Sissi') of Austria.  She was also the great-grandchild of King Leopold II of Belgium.

Archduchess Elisabeth Marie of Austria: Archduchess Elisabeth Marie of Austria (Elisabeth Maria Henriette Stephanie Gisela; 2 September 1883 – 16 March 1963) was the only child of Rudolf, Crown Prince of Austria and Princess Stéphanie of Belgium, and a granddaughter of Emperor Franz Joseph I of Austria and King Leopold II of the Belgians.  She was known to the family as "Erzsi", a diminutive of her name in Hungarian.  Later nicknamed "The Red Archduchess", she was famous for becoming a socialist and a member of the Austrian Social Democratic Party.

Princess Elisabeth of Luxembourg (1901–1950): Princess Elisabeth of Luxembourg (Elisabeth Marie Wilhelmine; 7 March 1901 – 2 August 1950) was the daughter of Grand Duke William IV and his wife Princess Adelheid-Marie of Anhalt-Dessau.

Princess Marie of the Netherlands: Princess Marie of the Netherlands (Dutch: "Prinses Wilhelmina Frederika Anna Elisabeth Marie der Nederlanden, Prinses van Oranje-Nassau" ; 5 June 184122 June 1910) was the fourth child and younger daughter of Prince Frederick of the Netherlands and wife of William, 5th Prince of Wied.  She was the mother of William, Prince of Albania.  She was the last surviving grandchild of William I of the Netherlands.

Stéphanie Windisch-Graetz: Stéphanie Windisch-Graetz (born 17 July 1939) is the daughter of Prince Franz Joseph zu Windisch-Graetz and granddaughter of Archduchess Elisabeth Marie of Austria.  Archduchess Elisabeth was the daughter of Rudolf, Crown Prince of Austria and granddaughter of Emperor Franz Joseph I of Austria.

Princess Stéphanie of Belgium: Princess Stéphanie of Belgium (21 May 1864 – 23 August 1945) was a Belgian princess by birth and became Crown Princess of Austria through her marriage to the heir-apparent of the Habsburg dynasty, Archduke Rudolf.  She was famously widowed in 1889 when Rudolf and his mistress, Mary Vetsera, were found dead in an apparent murder-suicide pact at the Imperial hunting lodge at Mayerling in the Vienna Woods.

Archduchess Hedwig of Austria: Archduchess Hedwig of Austria (Hedwig Maria Immakulata Michaela Ignatia; 24 September 1896 in Bad Ischl – 1 November 1970 in Hall in Tirol) was the second daughter of Archduke Franz Salvator of Austria and his wife, Marie Valerie of Austria.  She was a granddaughter of Emperor Franz Joseph I and Duchess Elisabeth in Bavaria.

Princess Elisabeth, Duchess of Hohenberg: Princess Elisabeth, Duchess von Hohenberg (Elisabeth Hilda Zita Marie Anna Antonia Friederike Wilhelmine Luise; 22 December 1922 – 22 November 2011) was a Luxembourgian princess.  She was a daughter of Grand Duchess Charlotte and Prince Felix of Bourbon-Parma, the sister of Jean, Grand Duke of Luxembourg and the aunt of Henri, Grand Duke of Luxembourg.  In 1956 she married Franz, Duke von Hohenberg, a grandson of Archduke Franz Ferdinand of Austria, whose assassination in 1914 sparked World War I.

Archduke Géza of Austria: Archduke Géza of Austria (given names: "Géza Ladislaus Euseb Gerhard Rafael Albert Maria"; born 14 November 1940, in Budapest), in Austria and Hungary only known by law as Géza Habsburg-Lothringen, is the son of Archduke Joseph Francis of Austria (1895–1957) and his wife Princess Anna of Saxony (1903–1976) thus he is a grandson of King Frederick Augustus III of Saxony and great-great grandson (on his father's mother's side) of Emperor Franz Joseph I of Austria and his wife Elisabeth of Bavaria (Sissi).  Géza is a Fabergé expert and has published many books and articles on the jewellers Peter Carl Fabergé and Victor Mayer.  He is the curator of several major international Fabergé exhibitions.

Princess Elisabeth Marie of Bavaria: Princess Elisabeth of Bavaria (German: "Elisabeth Marie Auguste Prinzessin von Bayern" ; 8 January 1874 – 4 March 1957) was a member of the Bavarian Royal House of Wittelsbach.  She was known as Princess of Bavaria till 1918.

Supporting paragraphs: Princess Stephanie of Windisch-Graetz Princess Stéphanie of Belgium
Answer: 21 May 1864