The ancestry of Severianus is unknown from contemporary sources. He is not known to have been count or duke of Cartagena. He is only said to have been a citizen. He might not even have lived in Cartagena. His wife's name is variously reported as Theodora, Theodosia, and Tutura. She is not documented as a daughter of Theodoric "the Great," king of the Ostrogoths.
Roger Collins, Visigothic Spain 409 - 711 (2008), p. 154 says: "All he [Isidore, Severianus' son] is saying is that Leander [another son] had been a monk in the province of Carthaginiensis not that he had been born there, let alone had to flee from it. So it is not necessary to believe that the family originated in Spain at all and as both Leander and Isidore had Greek names, it could be that their lost homeland lay outside the Iberian peninsula, possibly in Africa. The third of the brothers, Fulgentius, who became bishop of Ecija, has a name most of whose other associations are African."
https://books.google.com/books?id=WlSu_iWzA9AC&pg=PA154&dq=...
Sir William Smith, Henry Wace & J. Murray, A Dictionary of Christian Biography, Literature, Sects and Doctrines (1882), p. 637 says: "All that is historically known of the origin of the famous family to which the three brothers Leander, Isidore, and Fulgentius, and their only sister Florentina belonged, is derived from the opening sentence in Isidore's life of Leander (De Vir. Ill. Cap. 41; Esp. Sagr. v. 463), and from the concluding chapter of Leander's Regula, or Libellus ad Florentinum (Esp. Sagr. ix. 355). The father of the three sons and Florentina was one Severianus".
Later on the same page: "The traditions which connect Severianus with the great Theodoric, though quite worthless as to details, may [emphasis] preserve the memory of a possible Gothic origin of the family".
Then at p. 305: "We know that his [Isidore of Seville's] father was Severianus, who has frequently been called prefect of Cartagena. Isidore's own words, however (De Vir. ill. 41), are "genitus patre Severiano Carthaginensis provinciae;" where he is speaking of his brother Leander."
Later at p. 305: "Much greater uncertainty attaches to Isidore's mother. She has even been called the daughter of Theodoric, and her name is given as Theodora by the bishop of Palencia Don Rodgrigo Sanchez (Florez, ix. 192); but by others as Theodosia, and the addition is made of Cervella or Cervilia. But others again, resting on a metaphorical passage from a letter of Leander to his sister, have called her Turtura."
https://books.google.com/books?id=xt0UAAAAQAAJ&pg=PA637&dq=...
