When I say there's a "very large departure from history" in this story, I'm not kidding.
In reality, Octavian and Cleopatra Selene were twenty-three years apart in age, so he was closer to her mother's age than her own. For the purposes of this story, however, when Octavian took Egypt in 30 B.C., he will be nineteen, not thirty-three, while Selene will still be ten.
Also, SPOILER ALERT SKIP TO NEXT CHAPTER IF YOU DON'T WANT TO KNOW:
In reality, Cleopatra Selene married Juba II, King of Mauretania, while Octavia remained married to Livia Drusilla until his death in A.D. 14. In the story, they marry each other.