Question: Princess Maria Gabriella of Savoy, is the middle daughter of Italy's last king, Umberto II, and Marie José of Belgium, the "May Queen", and a sister of which pretender to their father's throne, the only son of Umberto II, the last King of Italy, and his wife, Marie-José of Belgium?
Context: Marina Doria is a Swiss water skier.  She is the wife of Vittorio Emanuele di Savoia, the son of the last king and queen of Italy, Umberto II and Princess Marie José of Belgium. Vittorio Emanuele of Savoy, Prince of Naples (Vittorio Emanuele Alberto Carlo Teodoro Umberto Bonifacio Amedeo Damiano Bernardino Gennaro Maria di Savoia; born 12 February 1937) is the only son of Umberto II, the last King of Italy, and his wife, Marie-José of Belgium.  Vittorio Emanuele also uses the title Duke of Savoy and claims the headship of the House of Savoy.  These claims are disputed by supporters of his third cousin, Amedeo, 5th Duke of Aosta. Princess Maria Pia of Bourbon-Parma (née Princess Maria Pia of Savoy; born 24 September 1934) is the eldest daughter of Umberto II of Italy and Marie-José of Belgium.  She is the older sister of Princess Maria Gabriella of Savoy, Vittorio Emanuele, Prince of Naples, and Princess Maria Beatrice of Savoy. Princess Maria Beatrice of Savoy (born 2 February 1944) is the youngest daughter of Italy's last King, Umberto II, and his wife, Queen Marie José. Princess Yolanda of Savoy (1 June 1901 – 16 October 1986) was the eldest daughter of King Victor Emmanuel III of Italy and his wife Queen Elena of Montenegro, and the sister of Umberto II, the last king of Italy. Marie José of Belgium (Marie-José Charlotte Sophie Amélie Henriette Gabrielle; 4 August 1906 – 27 January 2001) was the last Queen of Italy.  Her 35-day tenure as queen consort earned her the affectionate nickname "the May Queen". Princess Maria of Savoy (Maria Francesca Anna Romana; 26 December 1914 – 7 December 2001) was the youngest daughter of Victor Emmanuel III of Italy and Elena of Montenegro.  She was a sister of Umberto II of Italy and of Queen Giovanna of Bulgaria. Princess Maria Gabriella of Savoy (Maria Gabriella Giuseppa Aldegonda Adelaide Ludovica Felicita Gennara; born 24 February 1940) is the middle daughter of Italy's last king, Umberto II, and Marie José of Belgium, the "May Queen", and a sister of the pretender to their father's throne, Vittorio Emanuele, Prince of Naples.  She is an historical writer. Umberto II (Italian: "Umberto Nicola Tommaso Giovanni Maria di Savoia" ; 15 September 190418 March 1983), was the last King of Italy.  He reigned for 34 days, from 9 May 1946 to 12 June 1946, although he had been "de facto" head of state since 1944, and was nicknamed the May King (Italian: Re di Maggio ). Emanuele Filiberto of Savoy, Prince of Venice (Emanuele Filiberto Umberto Reza Ciro René Maria di Savoia; born 22 June 1972), is a television personality, a member of the House of Savoy and the son and heir of Vittorio Emanuele, Prince of Naples.  He is the only male-line grandson of Umberto II, the last King of Italy.  As one of the two heirs of the disputed headship of the House of Savoy, Emanuele Filiberto styles himself as "Prince of Piedmont".  However, due to the dynastic dispute, his right to the title of Prince of Piedmont is disputed.
Answer: Vittorio Emanuele of Savoy, Prince of Naples