Question: Who is Prince Friedrich Sigismund Of Prussia (1891–1927)'s maternal grandfather?

Princess Charlotte of Prussia (1831–1855)
Princess Charlotte of Prussia( 21 June 1831 – 30 March 1855), was by birth a Princess of Prussia and member of the House of Hohenzollern and by marriage Hereditary Princess of Saxe- Meiningen.
She was the eldest child and daughter of Prince Albert of Prussia( younger son of Frederick William III of Prussia and Louise of Mecklenburg- Strelitz) and his first wife Princess Marianne of the Netherlands( daughter of William I of the Netherlands and Wilhelmine of Prussia).
Princess Louise Sophie of Schleswig-Holstein-Sonderburg-Augustenburg
Princess Louise Sophie of Schleswig-Holstein-Sonderburg-Augustenburg
(German: Feodora Luise Sophie Adelheid Henriette Amalie; 8 April 1866 - 28 April 1952) was a daughter of Frederick VIII, Duke of Schleswig-Holstein and Princess Adelheid of Hohenlohe-Langenburg.
Princess Marianne of Prussia
Princess Marianne of Prussia( 23 August 19131 March 1983) was a member of the House of Hohenzollern, daughter of Prince Friedrich Wilhelm of Prussia.
Prince Friedrich Wilhelm of Prussia
Prince Friedrich Wilhelm of Prussia( 12 July 18809 March 1925) was a member of the House of Hohenzollern, great- grandson of Frederick William III of Prussia.
Prince Friedrich Karl of Prussia (1828–1885)
Prince Friedrich Karl of Prussia (20 March 1828 – 15 June 1885) was the son of Prince Charles of Prussia (1801–1883) and his wife, Princess Marie of Saxe-Weimar-Eisenach (1808–1877).
Prince Friedrich Karl was a grandson of King Frederick William III of Prussia and a nephew of Frederick William IV and William I.
He was born at Schloss Klein in Berlin.
As a military commander, the Prince had a major influence on the Royal Prussian Army's advances in training and tactics in the 1850s and 1860s.
He defeated the Austrian army at the Battle of Königgrätz in 1866 and the French Army of the Rhine at the Battle of Mars-la-Tour, overseeing the extinction of the Army of the Rhine at the Siege of Metz in 1870.
Charles Pickthorn
Major Charles Edward Murray Pickthorn( 20 September 1896 – 3 March 1938) was a British World
War I flying ace credited with five aerial victories.
His fourth victory resulted in the death of Prince Friedrich Karl of Prussia.
Prince Wilhelm of Prussia (1783–1851)
Prince Friedrich Wilhelm Karl of Prussia( July 1783, Berlin – 28 September 1851, Berlin) was the son of Frederick William II of Prussia and Frederika Louisa of Hesse- Darmstadt.
Prince Alexander of Prussia
Prince Friedrich Wilhelm Ludwig" Alexander" of Prussia( 21 June 1820 – 4 January 1896) was the eldest child of Prince Frederick of Prussia and his wife, Princess Luise of Anhalt- Bernburg.
Prince Friedrich Sigismund of Prussia (1891–1927)
Prince Joachim Viktor Wilhelm Leopold Friedrich Sigismund of Prussia (17 December 1891 – 6 July 1927) was a German World War
I fighter pilot and member of the House of Hohenzollern.
He was the son of Prince Friedrich Leopold of Prussia and Princess Louise Sophie of Schleswig-Holstein-Sonderburg-Augustenburg, married in 1916.
Prince Augustus of Prussia
Prince Friedrich Wilhelm Heinrich August of Prussia( 19 September 1779- 19 July 1843), known in English as Prince Augustus, was a Prussian general.
Born on Friedrichsfelde Palace, he was the youngest son of Prince Augustus Ferdinand of Prussia, the brother of King Frederick the Great, and Margravine Elisabeth Louise of Brandenburg- Schwedt.

Answer: Frederick VIII, Duke of Schleswig-Holstein