In my opinion, Brett and Chet aren't as bad as Isabela. I think Onceler, Brett and Chet were close as kids. But one day their father left, leaving their mother alone with her three children. Onceler was very similar to his father, which is why Isabela made him the outcast of the family. She encouraged Griselda and Ubb to do so. And the only people who didn't understand what was going on and still treated Onceler well were his brothers. Isabela didn't like it and every day she encouraged the boys to treat Onceler badly, saying it was nothing. Like little children, Brett and Chet began to listen to their mother. They weren't terrible, but they made Onceler feel bad every day. Brett and Chet often slap Onceler and stuff like that, but that's because they show him love. Because no one showed these three how to show love. They didn't have a good example. Despite this, Brett and Chet still love Onceler, but they also want to please their mother because they want her attention. The three of them had to be doing something to get her attention.
At the end when Onceler's family leaves, Brett and Chet look like they care about Onceler and they would like to stay with him, but they don't say anything because they don't want to oppose Isabela. When Isabela says that Brett is her favorite child now, Chet looks at it like it's unfair but doesn't say anything. In the deleted scenes the twins are actually worse, but in the movie they are not that bad.
In my opinion, Isabela simply screwed up as a mother and couldn't raise her three sons properly. That's my opinion.
Chet's reaction:
I also think Chet is an unwanted child too just like Onceler.
