Sancho Garcés, Lord of Uncastillo

[3] Sancho Garcés was an illegitimate son of King García Sánchez III and a concubine, born around 1038 and before his father married Stephanie.

In 1083, he was a member of an army under the command of his brother Ramiro and count Gonzalo Salvadórez tasked by Alfonso VI with accepting the capitulation of the rebel Muslim castle of Rueda.

In what became known as the 'disaster' or 'treachery of Rueda', the Castilian troops entered the surrendered fortress on 6 January 1083 only to be set upon by the garrison, who pelted them with stones, killing Sancho, Ramiro, count Gonzalo and many other nobles.

[4] He married his wife, Constanza, before 25 November 1057 as indicated in a document from the Monasterio of San Prudencio de Monte Laturce when both confirm a sale made by his brother the king.

In a charter dated 29 November 1074 recorded in the cartulary of the Monastery of Santa María de Otero de las Dueñas,[9][10] King Sancho Garcés IV gives his brother some houses and land in Calahorra, declaring: vobis germano meo domno Sancio et uxori vestra vel germana mea domna Constancia, that is, "to you, my brother Sancho, and to your wife and my sister, doña Constanza".