Complex sentence: There were claims that the attack was organized with the help of Carlos-Hugo's younger brother, Sixto Enrique de Borbón, who opposed Carlos Hugo's alteration of Carlism from an ultra-traditionalist political movement into a socialist movement.
Split into simpler sentences: There were claims that the attack was organized with the help of Carlos-Hugo's younger brother, Sixto Enrique de Borbón. He opposed Carlos Hugo's alteration of Carlism from an ultra-traditionalist political movement into a socialist movement.