Interestingly enough, I do not believe it is ever stated directly in the book which sister is older, but I think it is clear that Ellie is older than Brenda. The two, who both attend "the consolidated high school" (Chapter 6), are obviously very close in age, but Ellie is by far the smarter and more dominant one. Ellie and Brenda always appear together in the story, and most often, Ellie's name is listed first. Also, upon close examination of their dialogue, it is evident that Ellie is the instigator who drives most of their conversation, with Brenda most often chiming in only as a whining echo. Ellie acts like the older of the two, and in looking at the manner in which they are introduced, there is no reason to assume that Brenda might be older.
Ellie is the more overtly manipulative of the two older sisters. It is Ellie who orchestrates the shopping trip with the Timmonses for herself and Brenda, and effectively wheedles their mother into letting them go. She also sweet-talks their mother into giving them money for the trip, her "voice...sweeter than a melted Mars Bar". Ellie corrects Brenda's grammar when she complains that their mother doesn't "want (them) to have no fun at all", then flatly ignores her sister when she protests at being corrected. She imperiously tells Brenda that it is her turn to wash the dishes, and jabs her with a spoon when she protestingly complies (Chapter 1).