``Constantius gave over to his most inveterate enemies, his own cousin, the Caesar, his sister's husband, the father of his niece, the man whose own sister he had himself married in earlier days''. Gallus was a son of Julius Constantius and his first wife Galla. She is assumed to be a full sister of Gallus. Julius Constantius was a son of Constantius Chlorus and Flavia Maximiana Theodora. He was a paternal half-brother of Constantine I. In 337, Constantine I died.
Julius Constantius
Constantine I