Hmmm..... GfS, a few problems of detail. Constantine saw a vision of a cross in the sky in 312, before the battle of Milvian Bridge, which was against a Roman rival, Maxentius. It was in Rome, not Istanbul, which was in any case called Byzantium then, before Constantine renamed it Constantinople in 330. The Turks in the 4th century were still in central Asia, present day Turkey was divided into a number of Greek speaking states, the successors of Alexander's empire.
Constantine was not a nice man. He boiled his wife in a hot bath on the orders of his mother. Mothers-in-law can be troublesome things.