Brenda Sue, Ellie, and Joyce Ann are three of Jess's sisters, none of whom he really enjoys associating with. Brenda and Ellie are older and the best word to describe them is "obnoxious." They don't spend time with Jess and the younger kids, or even really notice them – that is, unless they see potential for mockery, complaining, or sensation. The two older sisters aren't imaginative, or interested in expanding their minds, and there's never any hint of an idea that they would be interested in being friends with Leslie. Here are the things they do like: shopping and boys.
Ellie is smarter than Brenda – she's the one who knows how best to manipulate their mother and sucker her into giving them money. She's also the one to take the lead and let Brenda know when enough's enough. Brenda's less clever and, perhaps because of this, meaner. When she tells Jess that Leslie is dead she almost seems to take pleasure in it. Previously in the book she'd made fun of Leslie for wearing boys' clothes and not acting feminine enough.
In contrast, Joyce Ann, the youngest, is treated by everybody as a baby, because she is. She's just too little to do very much, or really be very interesting. However, she's open and impressionable whereas Ellie and Brenda are harder and formed. This might be why Jess says Joyce Ann could maybe come to Terabithia, several years in the future, but never considers telling Brenda or Ellie about it.