One particularly interesting element of Nettie's letters to Celie is the way the use of language in Nettie's letters compared to that of Celie's as well as the focus Nettie applies to events.
Nettie, the more educated sister, uses a different vocabulary than Celie and a slightly different grammar. Where Celie uses cockney verbs or dialectic verb tenses, Nettie's uses "proper" grammar, fully conjugating verb tenses.
In Nettie's view of events, she focuses on victimization though she is not the victim. This is a contrast to Celie's early letters where she is often times abused and victimized. Nettie is more often in a position to report on abuses suffered by others.
In this way, Nettie is defined as an outsider in relation to Celie. Celie's letters keep Celie at the center of action, relating all events and relationships to her. Nettie's letters tend toward greater objectivity while also conveying a story of 1) foreign lands and cultures and 2) the "outsider's" perspective.
Nettie exists as the outsider in Samuel's family. She exists outside of American culture as well. The perspective she articulates in her letters mimics this status.