One of the most impressive strengths of Kingsolver's The Poisonwood Bible is that she writes it from the actual voices of 5 different women, and does such a good job of it that you can open up to any chapter in the book and guess which character is speaking simply from the way they speak. She uses a different voice for each girl, and gives each girl's narration style its own traits that set it apart from the others.
For example, Rachel always mispronounces and misuses words. She does this in every chapter that she narrates, so if you open up to a section and see a rather headstrong and snobby girl that is whining and misusing words, it's bound to be Rachel. Here's just a few of her misused words: "executrate" (execute), "autography" (autobiography), "Morse Scold" (Morse Code), and "preciptation" (participation). Rachel is also whiney, petulant, cynical, judgmental and sarcastic. Kingsolver used all of those traits to set her apart.
Adah's chapters all show her fascination with palindromes, or words that spelled backwards are the same, and take on a symbolic meaning. She also quotes poetry, and has a secret backwards code that she uses quite a bit. She is also very dark, sardonic and cryptic. She is self-deprecating and constantly demeaning her importance and emphasizing her crippled stature.
Leah is open, frank, sincere and intense. She feels passionately about things, and focuses more on the issues of Africa and the injustices that she sees. She is less critical of her father, and wants to fit in with him and also with the African boys and culture around her. Her openness is a key to her voice, as is her vigorous and intent nature.
Ruth May is easy to pick out because her narration is child-like, her phrases simple, and everything is infused with an innocent and touching naivety and child-like perspective.
Kingsolver uses voice to set apart the narration from each of the Price sisters, and gives each one of them distinct traits and quirks that make them all separate and unique. It is quite a feat to accomplish, and one that draws the readers into the book even more. I hope that helps a bit; good luck!