On his deathbed? Wasn’t his wife also very influential is his conversion?
He executed Fausta, his wife, some years earlier, by locking her in an overheated bath. They think she had been conspiring against the Emperor with his son, Crispus, whose head he had cut off.
You might be thinking of his mother, Saint Helena, former concubine of the Emperor Constantius I. She died seven years before her son.