Princess Barbara of Prussia
Princess Barbara of Prussia (2 August 192031 May 1994) was the only daughter and elder child of Prince Sigismund of Prussia and Princess Charlotte of Saxe-Altenburg.
She was a great-granddaughter of Frederick III, German Emperor and a great-great-granddaughter of Queen Victoria.
On 17 September 1952 she was legally adopted by her paternal grandmother, Princess Irene of Prussia ("née" Princess Irene of Hesse and by Rhine), a granddaughter of Queen Victoria.
Princess Marie of Prussia (1855–1888)
Princess Marie of Prussia(" Marie Elisabeth Louise Frederika"; 14 September 1855, Marmorpalais, Potsdam – 20 June 1888, Dresden), was a princess of the House of Hohenzollern.
She was the daughter of Prince Frederick Charles of Prussia and later became second wife of Prince Henry of the Netherlands then the first wife of Prince Albert of Saxe- Altenburg.
She was also the great- niece of Wilhelm I, German Emperor.
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.
Princess Elisabeth of Prussia
Princess Elisabeth of Prussia( 18 June 1815 – 21 March 1885) was the second daughter of Prince Wilhelm of Prussia and Landgravine Marie Anna of Hesse- Homburg and a granddaughter of Frederick William II of Prussia.
Prince Philip, Duke of Edinburgh is her great- great- grandson.
Princess Irene of Hesse and by Rhine
Princess Irene of Hesse and by Rhine ("Irene Luise Marie Anne, Princess of Hesse and by Rhine", 11 July 1866 – 11 November 1953) was the third child and third daughter of Princess Alice of the United Kingdom and Louis IV, Grand Duke of Hesse and by Rhine.
Her maternal grandparents were Queen Victoria and Prince Albert of Saxe-Coburg and Gotha.
Her paternal grandparents were Prince Charles of Hesse and by Rhine and Princess Elizabeth of Prussia.
She was the wife of Prince Henry of Prussia, a younger brother of Wilhelm II, German Emperor and her first cousin.
The SS "Prinzessin Irene", a liner of the North German Lloyd was named after her.
Her siblings included Princess Victoria of Hesse and by Rhine, wife of Prince Louis of Battenberg, Grand Duchess Elizabeth Feodorovna of Russia, wife of Grand Duke Sergei Alexandrovich of Russia, Ernest Louis, Grand Duke of Hesse and by Rhine, and Empress Alexandra Feodorovna of Russia, wife of Tsar Nicholas II of Russia.
Like her younger sister, the empress, Irene was a carrier of the hemophilia gene, and Irene would lose her sisters Alix and Elisabeth in Russia to the Bolsheviks.
Prince Sigismund of Prussia (1896–1978)
Prince William Victor Charles Augustus Henry Sigismund of Prussia (27 November 1896 at Kiel – 14 November 1978 at Esparza, Costa Rica), was the second son of Prince Henry of Prussia and his wife, Princess Irene of Hesse and by Rhine.
He was a nephew of Kaiser Wilhelm II and Tsarina Alexandra of Russia.
A great-grandson of Queen Victoria through both his parents
, he was the only one of three brothers who did not have the hemophilia common among her descendants.
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.
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 Calixta of Lippe
Princess "Calixta" Agnes Adelaide Irmgard Helene Caroline Elise Emma of Lippe (14 October 1895 - 15 December 1982) was the wife of Prince Waldemar of Prussia, eldest son of Prince Henry of Prussia.
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.

Question: Who is Prince Sigismund Of Prussia (1896–1978)'s maternal grandfather?
Answer: Louis IV, Grand Duke of Hesse