She Who Became the Sun by Shelley Parker-Chan
In 14th century China, a girl is born into famine. The local soothsayer says that her brother, Zhu Chongba, will live a life of greatness, while her future is empty. She will be nothing. So when her brother dies in the famine, she takes over his life, hoping to trick Heaven and also take over his greatness. She disguises herself as a boy and becomes a monk, then talks her way into the army, where her cleverness and political savvy have her swiftly climbing the ranks.