Princess Helena of Waldeck and Pyrmont (1899–1948)
Princess Helena of Waldeck and Pyrmont (22 December 189918 February 1948) was the only daughter of Friedrich, Prince of Waldeck and Pyrmont, last reigning Prince of Waldeck and Pyrmont, and wife of Nikolaus, Hereditary Grand Duke of Oldenburg.
Princess Helena of Waldeck and Pyrmont
Princess Helena of Waldeck and Pyrmont( Helene Friederike Auguste; later Duchess of Albany; 17 February 1861 – 1 September 1922), who became a member of the British royal family by marriage, was the fifth daughter and child of George Victor, Prince of Waldeck and Pyrmont and his first wife, Princess Helena of Nassau.
Princess Mathilde of Waldeck and Pyrmont
Princess" Mathilde" of Waldeck and Pyrmont( 10 April 1801 – 13 April 1825) was a member of the House of Waldeck and Pyrmont and a Princess of Waldeck and Pyrmont and a member of the House of Württemberg and a Duchess of Württemberg through her marriage to Duke Eugen of Württemberg
Princess Caroline of Waldeck and Pyrmont
Caroline Louise of Waldeck and Pyrmont( 14 August 1748 – 18 August 1782), was a Princess of Waldeck and Pyrmont by birth and Duchess consort of Courland by marriage.
She was the daughter of Karl August, Prince of Waldeck and Pyrmont, and Countess Palatine Christiane Henriette of Zweibrücken- Birkenfeld.
George I, Prince of Waldeck and Pyrmont
George I, Prince of Waldeck and Pyrmont( 6 May 17479 September 1813) was Prince of Waldeck and Pyrmont from 1812 to 1813.
He was the son of Karl August, Prince of Waldeck and Pyrmont and Countess Palatine Christiane Henriette of Zweibrücken- Birkenfeld.
George II, Prince of Waldeck and Pyrmont
George II, Prince of Waldeck and Pyrmont( 20 September 178915 May 1845) was Prince of Waldeck and Pyrmont from 1813 to 1845.
He was the son of George I, Prince of Waldeck and Pyrmont and Countess Princess Augusta of Schwarzburg- Sondershausen.
Princess Marie of Waldeck and Pyrmont
Princess Marie of Waldeck and Pyrmont (Georgine Henriette Marie; 23 May 1857 – 30 April 1882) was the third daughter of George Victor, Prince of Waldeck and Pyrmont and his wife, Princess Helena of Nassau, younger half-sister of Adolphe, Grand Duke of Luxembourg.
Nikolaus, Hereditary Grand Duke of Oldenburg
Nikolaus, Hereditary Grand Duke of Oldenburg (10 August 18973 April 1970) was the eldest son of Frederick Augustus II, Grand Duke of Oldenburg, who was the last ruling Grand Duke of Oldenburg.
In 1931, Nikolaus succeeded to his father's titles and assumed the role of pretender to the Grand Duchy, until his death in 1970.
Wittekind, Prince of Waldeck and Pyrmont
Wittekind Adolf Heinrich Georg- Wilhelm, Prince of Waldeck and Pyrmont( born 9 March 1936) has been since 1967 the head of the House of Waldeck and Pyrmont.
Princess Guda of Waldeck and Pyrmont
Princess Guda of Waldeck and Pyrmont( born 22 August 1939) is the youngest daughter of Josias, Hereditary Prince of Waldeck and Pyrmont, head of the House of Waldeck and Pyrmont from 1946 to 1967 and first wife of Frederick William, Prince of Wied.

Question: Who is Princess Helena Of Waldeck And Pyrmont (1899–1948)'s father-in-law?
Answer: Frederick Augustus II, Grand Duke of Oldenburg