Alphabet of Dreams by Susan Fletcher 294 p.

Orphans Mitra and Babak are barely surviving in the caves of a Persian city. Mitra, disguised as a boy, steals what food she can for her sensitive younger brother, but she yearns to return to the prosperous and happy life they once led as children of royalty. When a Magus discovers that Babak can predict the future with his dreams, the two children are taken on a journey through dangerous Roman country, following the stars and awaiting the birth of a special child.