Poisoned Empire by Elyse Thomson

Cinderella (feral) meets The Lies of Locke Lamora in this lush romantic fantasy set in a world inspired by the Eastern Roman Empire

Black-marketeer Selene has poison magic and the cynicism to match. When she and genius metals mage Iliana are arrested by the same scheming, noble fathers who tossed them out at birth, they suspect apologies won’t be forthcoming. Forced to either impersonate their half-sisters or die, the friends are stuffed into fancy dresses, packed off to the capital, and thrust into the perilous, glittering world of the imperial court.
Traitors lurk in Prince Belisarius’ court, and only his loyal strategos Marduk is above suspicion. As noble-born villains siphon away the souls of their daughters to magnify their magic in secret, Belisarius plots to expose them all—by inviting every noblewoman in the empire to compete for his hand in marriage. But two infuriating imposters in attendance quickly become his bane.
When the friends are discovered, they expect imprisonment—not a deal. Vast riches are on offer if Selene poses as fiancée to the handsome prince while Iliana simpers for the towering strategos—a ploy to lure traitorous enemies to the capital. Yet even as they help secure the throne, false affections flirt with real passions, and Selene and Iliana are convinced they’ll either lose their hearts… or their heads.