Kleopatra Selene, her twin, Alexander, and their younger brother Ptolemy suffer the loss of their parents, Kleopatra and Marc Antony, when Romans invade Egypt. After the suicides of the King and Queen of Egypt, Selene and her brothers are taken to Rome, where they are paraded through the streets as spoils of war at age 11. 
Octavian is the ruler of Rome and he has the children live with his sister and her son Marcellus, and their half sisters Antonia and Tonia, in real life both were named Antonia, apparently Romans didn't like to vary much if at all on names. I'm glad the author took creative liberties here, otherwise this could have gotten confusing.
They are very well educated children with vivid personalities based on historical records, though in the book hers is clearly the dominant one. Selene is intelligent, strong-willed and compassionate making her interesting to follow. Initially she's a bit bratty but she grows over the 5 years that this story takes place in.