Kingsolver's novel is not only a story of a family and their journey over several decades, it is also a political allegory for Africa's journey itself. Kingsolver brilliantly weaves underlying historical approaches into the characters of the daughters themselves. If you look closely at their characters, their nicknames and symbols attached to them, you can connect each one of them to a different type of political or historical example to the country.
Leah is a strong character who becomes assimilated into the culture of Africa. She learns to love it, stays there, and does her best to try to better the country through living there and raising a strong family. This compares to the approach of others who offer aid to a struggling country, but not just in money or gifts, but in their lives. These are the people that come to the country and live there themselves, and in the act of doing so, learn to love it with all of their hearts.
Ada represents how an experience with another culture or country can be life-changing and strengthening. She went to Africa a cripple; she leaves a cripple also, but she is the only one that makes it out, and eventually is healed. Her experience there changed her for the better--this is an example of the profound changes that many experience in other cultures; they then take those changes and bring them back with them, bettering the world they live in with them.
Rachel represents those that take advantage, manipulate and glean wealth from the conquering of other countries. She goes to Africa and never learns to love it; instead, she hates it and the people until her dying day. She runs a hotel and continues to treat the locals like foreigners, using them as labor to bring other "normal" people over. She symbolizes the colonization and often brutal mistreatment of local cultures by a dominant overtaker.
Ruth May represents the awful sacrifices that are made in order to conquer another race or culture. When her father tries to change the customs of the tribe, and to override the authority of the chief, Ruth May is the victim. It is the same when anyone tries to conquer another people; in the act, innocents are sacrificed.
Orleanna herself is harder to pinpoint, but I would say that she is a symbol for the country itself; stripped of all that is her own, mourning the losses eternally, forever scarred and marked by the wills and passions of men. It is her husband (also a symbol of cruel colonization tactics) that drags her there; she does her best to survive but in the end is ravaged by his strong will and the sacrifices demanded.
I hope that those thoughts help to get you started; good luck!