He is mentioned in the cartulary of Auch, as a son of García Sánchez, and the Codex of Roda, which mentions him as the heir of Gascony. On his father's death, he inherited the duchy itself, which included the viscounties of Lomagen, Gavarret, Tursan, and Bruillois. His younger brothers, William and Arnold, inherited Fézensac (including Armagnac) and Astarac respectively with the comital title. Thus was Gascony divided and diminished.
In 932, Flodoard records that the brothers Ermengol of Rouergue and Raymond Pons of Toulouse, Princes of Gothia, brought a "Lupus Aznar Vasco" with them to do homage to Rudolph of France. Lewis considers this "Vasco" to be the duke of Gascony and calls him "Sánchez." The duke meant, however, would be Sancho.
Sancho had two illegitimate sons, Sancho, who later succeeded him, and William, who likewise succeeded his childless brother. A third son was Gombald, a noted pluralist bishop, who held the various sees of Gascony as one until being reappointed to the long-vacant archdiocese of Bordeaux. A fourth son Udalrich or Odulric is attested in charters; probably all were bastards.