Although the characterization in both story, screenplay, and movie are superb, there remains a few shadowy characters whose disposition and temperament remain largely unknown.
Feel free to cite them here and to discuss then: speculation as to their natures is allowed.
Let me start with Ennis's sister and brother. As adults, they only show up only once in the film (the wedding) and as a child, the brother only shows up once (the Earl and Rich episode).
Thery strike me as completely different from Ennis. THe sister is probably more worldly, electing to abandon the siblings early on (so as not to be a brudern to them?), marry, and move to Casper.
The brother is an enigma. He must have wanted to free himself from being Ennis's caretaker. I had always found it strange that he let go of Ennis after his marriage. I would suppose that in rural USA, family bonds are stronger. But I speculate here...