I think the tranformation and moral growth of Leah can be mapped out by investigating her relationship with her father. At the beginning of the novel we see that she is wholeheartedly supportive of him. She helps him re-sow the seeds that have been dug up by their helper and, out of all of her sisters, is the one that wants to spend time withhim. However, as time goes on, Leah begins to realise the truth about her father and his mission and the way that he is so misguided in so many ways. Her future is determiend by her experiences in the Congo and her passionate commitment to fight for justice.
The above post is a response to Adah's transformation in the novel. Her twin sister Leah changes her perspective as the novel progresses. When the Price family first arrives in the Congo on their mission, Leah believes in her father's quest to convert the villagers. However, as he continues in this effort, Leah begins to see that her father's motives are more driven by his ego rather than a true belief in the religion. As a result, Leah stops supporting her father and gives up their religion. She falls in love with Anatole and stay with him in the Congo to work on behalf of struggling villagers.
The most evident (while maybe not the most important)transformation is that she is no longer a cripple. She is intelligent from the beginning, and very observant. She learns more about herself and her family through watching them in situations than through any other means. Her relationship with her mother also transforms as she recognizes and realizes the reasons for her mother's choices. It is only after her return to the USA that she is able to walk without trouble and that she begins speaking as her twin has always done all along.