I was thinking maybe the city fell to civil war after the death of her father. Two factions arose, one supporting her brother, a man with a reputation for being ruthless and hedonistic, and one supporting Dehana, the more diplomatic and reasonable of the two. The support of the nobles and the warlords went to her brother though, for they thought him to be the more easily controlled of the bunch. Eventually, the better armed and better trained side won out, forcing Dehana from her home as she sought to avoid capture. Her brother wants her alive, either to humiliate her by taking her as his own wife or selling her to some foreign warlord. Her quest will probably be one of gathering strength and trying to convince her father's allies that she is the 'rightful' ruler of her city, or at least convince them that her brother is too dangerous to leave in power.