Duchess Therese of Mecklenburg-Strelitz
Duchess Therese Mathilde Amalie of Mecklenburg-Strelitz (5 April 1773 – 12 February 1839) was a member of the House of Mecklenburg-Strelitz and a Duchess of Mecklenburg.
Through her marriage to Karl Alexander, 5th Prince of Thurn and Taxis, Therese was also a member of the House of Thurn and Taxis.
Duchess Gustave Caroline of Mecklenburg-Strelitz
Duchess Gustave Caroline of Mecklenburg- Strelitz( 12 July 1694 – 13 April 1748) was a daughter of Adolphus Frederick II, Duke of Mecklenburg and Princess Maria of Mecklenburg.
Georg Alexander, Duke of Mecklenburg
Georg Alexander, Duke of Mecklenburg( 27 August 1921 – 26 January 1996) was the head of the House of Mecklenburg- Strelitz from 1963 until his death.
Duke Georg August of Mecklenburg-Strelitz
Georg August( 11 January 1824 – 20 June 1876) was a member of the House of Mecklenburg- Strelitz and titular duke of Mecklenburg- Strelitz.
Grand Duchess Catherine Mikhailovna of Russia
Grand Duchess Catherine Mikhailovna of Russia  (28 August 1827 – 12 May 1894), was the third of five daughters of Grand Duke Michael Pavlovich of Russia (youngest son of Emperor
Paul I and Princess Sophie Dorothea of Württemberg) and Princess Charlotte of Württemberg (eldest child of Prince Paul of Württemberg and Princess Charlotte of Saxe-Hildburghausen).
Duchess Helene of Mecklenburg-Strelitz
Duchess Helene of Mecklenburg-Strelitz (16 January 1857 – 28 August 1936) was a daughter of Duke Georg August of Mecklenburg-Strelitz and his wife, Grand Duchess Catherine Mikhailovna of Russia.
She was the second wife of Prince Albert of Saxe-Altenburg.
Duke Carl Gregor of Mecklenburg
Carl Gregor Duke of Mecklenburg( 14 March 1933 – 23 July 2018) was a member of the House of Mecklenburg- Strelitz and a music and art historian.
Princess Elisabeth of Anhalt
Princess Elisabeth of Anhalt (7 September 1857 – 20 July 1933) was the Grand Duchess of Mecklenburg-Strelitz from 1904 to 1914 as the spouse of Adolf Friedrich V, Grand Duke of Mecklenburg-Strelitz.
Duchess Charlotte Georgine of Mecklenburg-Strelitz
Duchess Charlotte Georgine Luise Friederike of Mecklenburg- Strelitz( 17 November 1769 – 14 May 1818) was a member of the House of Mecklenburg- Strelitz and a Duchess of Mecklenburg- Strelitz by birth and a Duchess of Saxe- Hildburghausen through her marriage to Frederick, Duke of Saxe- Hildburghausen( later Duke of Saxe- Altenburg).
Princess Augusta of Cambridge
Princess Augusta of Cambridge( 19 July 1822 – 5 December 1916) was a member of the British Royal Family, a granddaughter of George III.
She married into the Grand Ducal House of Mecklenburg- Strelitz and became the Grand Duchess of Mecklenburg- Strelitz.

Question: Who is the maternal grandfather of Duchess Helene Of Mecklenburg-Strelitz?