The two sisters setting has elements of the fantasy genre of historical fantasy and the science fiction genre of alternative history. The setting is a timeline where the Roman Empire over came its challenges and never fell. The Roman Imperium did become two states in the 3rd century, just as in our own world, but the political development was radically different.

The first major change was the land reform of Tiberius Gracchus succeeded so the Roman Republic did not go through the throes of civil war. It remained a powerful republic with dominions and client states well into the first century CE.

The next major difference was the Roman conquest of the Alexanderian successor states was more though, with the Eastern wars being more brutal and the Roman Imperium extending almost to the Indus River and the Caspian Sea as a result.

The Roman conquest of the Celtic world would be slower, but would project all the way to Eire. The Germanic conquest would also be much later than in our timeline, but would project well to the River Elbe and eventually the Jutland Peninsula.

All of this vast territory would fragment the Roman Imperium. The Imperium was split into a Western and Eastern partitions.  The East would become more imperialistic, building upon the traditions of Persia and Macedonia, while the West would build upon the Republican roots of Rome and the individualism of the Celtic and Germanic tribes. The West would leave Rome as the cultural capital but would build great commerce centers at Carthage, Londinium, Cologne, and Athens. The East used Babylon as their capital, but still built heavily at Bzantinium, Alexandria, Antioch, Ecbatana, and Thatta on the Indus.