Archduke Joseph August of Austria
Archduke Joseph August Viktor Klemens Maria of Austria, Prince of Hungary and Bohemia (9 August 1872 – 6 July 1962) was a "Feldmarschall" (Field Marshal) of the Austro-Hungarian Army and for a short period head of state of Hungary.
He was a member of the House of Habsburg-Lorraine, the eldest son of Archduke Joseph Karl of Austria (1833–1905) and his wife Princess Clotilde of Saxe-Coburg and Gotha (1846–1927).
Joseph August's grandfather had been Palatine Joseph of Hungary (1776–1847), Palatine and Viceroy of Hungary, a younger son of Emperor Leopold II.
The Archduke Joseph Diamond, a 76.02 carat colourless diamond with internal flawless clarity, is named after the Archduke and officially recorded as his property.
Cornelia (mother of the Gracchi)
Cornelia( c. 190s – c. 115 BC) was the second daughter of Publius Cornelius Scipio Africanus, the hero of the Second Punic War, and Aemilia Paulla.
She is remembered as a prototypical example of a virtuous Roman woman.
She was the mother of the Gracchi brothers, and the mother- in- law of Scipio Aemilianus.
Priscilla Pointer
Priscilla Marie Pointer( born May 18, 1924) is an American stage, film and television character actress.
She began her career in the theater, including productions on Broadway.
Later, Pointer moved to Hollywood to act in films and on television.
She is the mother of Amy Irving, therefore making her the former mother- in- law of filmmakers Steven Spielberg and Bruno Barreto and the mother- in- law of documentary filmmaker Kenneth Bowser, Jr.
Marian Shields Robinson
Marian Lois Robinson( née Shields; born July 29, 1937) is the mother of Michelle Obama, former First Lady of the United States, and Craig Robinson, a basketball executive, and the mother- in- law of former U.S. President Barack Obama.
Baroness Gösta von dem Bussche-Haddenhausen
Baroness Gösta von dem Bussche- Haddenhausen( 26 January 1902 – 13 June 1996) was the mother of Prince Claus of the Netherlands, who was the Prince Consort of Queen Beatrix of the Netherlands, thus making her the mother- in- law of the former Dutch Queen.
She is also the paternal grandmother of King Willem- Alexander of the Netherlands, who is the current Dutch King.
Eldon Howard
Eldon Howard was a British screenwriter.
She was the mother- in- law of Edward J. Danziger and wrote a number of the screenplays for films by his company Danziger Productions.
Ebba Eriksdotter Vasa
Ebba Eriksdotter Vasa (circa 1491 – 21 November 1549) was a Swedish noblewoman.
She was the mother of Queen Margaret Leijonhufvud and the second cousin and mother-in-law of King Gustav Vasa.
Princess Auguste of Bavaria (1875–1964)
Princess Auguste of Bavaria  (28 April 1877 – 25 June 1964) was a member of the Bavarian Royal House of Wittelsbach and the spouse of Archduke Joseph August of Austria.
Maria Thins
Maria Thins( c. 1593 – 27 December 1680) was the mother- in- law of Johannes Vermeer and a member of the Gouda Thins family.
Vera Miletić
Vera Miletić (Serbian Cyrillic: Вера Милетић; 8 March 1920 – 7 September 1944) was a Serbian student and soldier.
She was notable for being the mother of Mira Marković, posthumously making her the mother-in-law of Serbian president Slobodan Milošević.

Question: Who is Princess Auguste Of Bavaria (1875–1964)'s mother-in-law?
Answer: Clotilde of Saxe-Coburg and Gotha