The Last Girls of Pompeii — Kathryn Lasky

In Pompeii, in the summer of A.D. 79, Julia and Sura appear to lead opposite lives. Julia is the daughter of a wealthy ship-builder; Sura is an orphan. Julia bears the Curse of Venus -- a withered arm; Sura's beauty turns heads. Julia is free; Sura is her slave.
Then Julia learns that her parents are planning to put her in the service of the Temple of Damia, the center of a cultish new religion, and Sura will be sold to an awful man who plans to make her his concubine.

But when Mt. Vesuvius erupts, Julia's and Sura's fates are forever altered, forcing them both to face the true meaning of freedom.