Prince Edward, Duke of Kent
Prince Edward, Duke of Kent, (Edward George Nicholas Paul Patrick; born 9 October 1935) is a member of the British royal family.
He is a first cousin of Queen Elizabeth II through their fathers, Prince George, Duke of Kent, and King George VI.
Because his mother, Princess Marina of Greece and Denmark was a cousin of Prince Philip, Duke of Edinburgh, Edward is both a second cousin and first cousin once removed to Prince Charles and his siblings.
He has held the title of Duke of Kent since the age of six, after the death of his father in a plane crash in 1942.
He carries out engagements on behalf of the Queen.
He is president of the All England Lawn Tennis and Croquet Club, presenting the trophies to the Wimbledon champion and runner-up, and served as the United Kingdom's Special Representative for International Trade and Investment, retiring in 2001.
He is president of The Scout Association, the Royal United Services Institute, and the Royal Institution of Great Britain, and since 1967 Grand Master of the United Grand Lodge of England.
He is also patron of the Institute of Advanced Motorists, an independent road safety charity which specialises in training and advice for post-licence drivers and riders.
George Windsor, Earl of St Andrews
George Philip Nicholas Windsor, Earl of St Andrews( born 26 June 1962) is the elder son of Prince Edward, Duke of Kent, and his wife Katharine, Duchess of Kent.
He is styled Earl of St Andrews, one of his father's subsidiary titles, which he holds by courtesy as heir apparent to the Dukedom of Kent.
Lady Helen Taylor
Lady Helen Marina Lucy Taylor ("née" Windsor; born 28 April 1964) is a relative of the British royal family.
She is the daughter of Prince Edward, Duke of Kent, and Katharine, Duchess of Kent, and a great-granddaughter of King George V. She is currently 43rd in line of succession to the British throne.
Lord Nicholas Windsor
Lord Nicholas Charles Edward Jonathan Windsor( born 25 July 1970) is a member of the British royal family, the youngest child of Prince Edward, Duke of Kent, and Katharine, Duchess of Kent, and a great- grandson of King George V.
He is a first cousin once removed of Queen Elizabeth II.
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.
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.
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.
Giles Worsley
Giles Arthington Worsley( 22 March 1961 – 17 January 2006) was an English architectural historian, author, editor, journalist and critic, specialising in British country houses.
He was the second son of Sir Marcus Worsley of Hovingham Hall, a nephew of Katharine, Duchess of Kent, and died of cancer aged 44.
Katharine, Duchess of Kent
Katharine, Duchess of Kent, (born Katharine Lucy Mary Worsley; 22 February 1933)
is a member of the British royal family.
Her husband, Prince Edward, Duke of Kent, is a first cousin of Queen Elizabeth II.
The Duchess of Kent gained attention for her conversion to Catholicism in 1994; she was the first member of the royal family to convert publicly since the passing of the Act of Settlement 1701.
The Duchess of Kent is strongly associated with the world of music and has performed as a member of several choirs.
Felix Brunner
Sir Felix John Morgan Brunner, 3rd Baronet( 13 October 1897 – 2 November 1982) was a British Liberal Party politician and business owner.
He was the maternal uncle of Katharine, Duchess of Kent.

Question: Who is the mother-in-law of Katharine, Duchess Of Kent?
Answer: Princess Marina of Greece and Denmark