He died without issue on 23 April 1766.
The 2nd Marquess of Villarías was his brother, Agustín de la Cuadra y de Llarena, (5 November 1679 - married 17 December 1709 to Andresa Manuela de Mollinedo).

Agustin and Sebastian had a sister, Francisca de la Cuadra y de Llarena, born also in San Julián de Musques on 13 October 1697.
She married Simon de Llano y Musquez on 16 February 1716; they had 8 children.
Among their children were:
Sebastián de Llano y la Cuadra, born in 1727, who married in The Hague, Holland, Baroness María de Adelmar Paniariui, in 1783.
He became the 3rd Marquess of Villarías in 1766 and was promoted to conde de Senafé (8 August 1780), awarded by King Charles III of Spain.
He was also Ambassador to the United Provinces under King Charles IV of Spain.

José Antonio de Llano y la Cuadra.
Ambassador in Vienna, Austria, He was promoted to Marqués de Llano on the death in 1794 of his brother José Agustín.

José Agustín de Llano y la Cuadra (15 October 1722 - 5 March 1794), born also in San Julián de Musques (today Muskiz, in Basque language).
Oficial Mayor to the First Secretary of State, Ambassador in Vienna, Austria, Knight of the Military Order of Santiago in 1741, (sig.
4729), 1st Marqués de Llano, (23 April 1772), by King Charles III of Spain.
He married on 12 June 1770 the very wealthy Isabel Parreño, from Corral de Almaguer, province of Toledo, Spain.
She was painted by the famous Danish-German Court painter Anton Raphael Mengs (1728–1779), and  protected in Vienna in 1785, Spanish opera and ballet composer Vicente Martín y Soler, (Valencia, 1754 - Saint Petersburg, Russia, January 1806)  monitoring him to become the Director, court composer, after 1788 of the Saint Petersburg Opera, with Empress Catherine II of Russia.
He died without issue on 23 April 1766. The 2nd Marquess of Villarías was his brother, Agustín de la Cuadra y de Llarena, (5 November 1679 - married 17 December 1709 to Andresa Manuela de Mollinedo).
Agustin and Sebastian had a sister, Francisca de la Cuadra y de Llarena, born also in San Julián de Musques on 13 October 1697. She married Simon de Llano y Musquez on 16 February 1716; they had 8 children. Among their children were:
Sebastián de Llano y la Cuadra, born in 1727, who married in The Hague, Holland, Baroness María de Adelmar Paniariui, in 1783. He became the 3rd Marquess of Villarías in 1766 and was promoted to conde de Senafé (8 August 1780), awarded by King Charles III of Spain. He was also Ambassador to the United Provinces under King Charles IV of Spain.
José Antonio de Llano y la Cuadra. Ambassador in Vienna, Austria, He was promoted to Marqués de Llano on the death in 1794 of his brother José Agustín.
José Agustín de Llano y la Cuadra (15 October 1722 - 5 March 1794), born also in San Julián de Musques (today Muskiz, in Basque language). Oficial Mayor to the First Secretary of State, Ambassador in Vienna, Austria, Knight of the Military Order of Santiago in 1741, (sig. 4729), 1st Marqués de Llano, (23 April 1772), by King Charles III of Spain. He married on 12 June 1770 the very wealthy Isabel Parreño, from Corral de Almaguer, province of Toledo, Spain. She was painted by the famous Danish-German Court painter Anton Raphael Mengs (1728–1779), and  protected in Vienna in 1785, Spanish opera and ballet composer Vicente Martín y Soler, (Valencia, 1754 - Saint Petersburg, Russia, January 1806)  monitoring him to become the Director, court composer, after 1788 of the Saint Petersburg Opera, with Empress Catherine II of Russia.
Sebastián de la Cuadra, 1st Marquess of Villarías