what motivates Cassie to act nicely toward Lillian Jean at the start of Chapter 8 of Roll of Thunder, Hear My Cry?

She wants to make her brothers angry. She wants to make life easier for Lillian Jean.

Explanation:

I can't personally speak for every single Dystopian Fiction author, but personally what I think about why they write it is because it allows them to explore what exactly earth could turn out to be. It reaches out to audiences and makes them think about how the world could get messed up, and it offers a pathway into that dystopian universe.