Theodora was the step-daughter of the emperor Maximianus, and thus putting aside Helena and marrying her (probably in about 289) made good political sense for Constantius Chlorus. Constantius became Caesar on his father-in-law's abdication. Theodora's marriage did, of course, gain her children and grand-children the enmity of St. Helena, who long outlived her rival.
Theodora bore Constantius six children, including a daughter Anastasia ("Resurrection"), whose name is offered as evidence by those proposing Constantius I Chlorus as a Christian, or at least a Christian sympathizer. Among her other children was a son Julius Constantius, the father of Gallus and Julian. She was half-sister to Constantine I's wife, Fausta.