Sandy and Dennys are twins, and might as well be clones for the amount of individual characterization they get. (They do get more differentiated in the sequels, and Many Waters is devoted to their adventures when they pay a visit to Noah, of ark fame.) Sandy and Dennys, as Meg enviously remarks to herself, are the most normal members of the Murry clan:
The twins didn't have any problems. They weren't great students, but they weren't bad ones, either. They were perfectly content with a succession of B's and an occasional A or C. They were strong and fast runners and good at games, and when cracks were made about anybody in the Murry family, they weren't made about Sandy and Dennys. (1.25)
Sandy and Dennys's comforting averageness means they don't set off the town's freak detectors like the rest of their family does, and so they get much less grief from their peers. While Sandy and Dennys don't get much to actually do in the book, they do serve as a potent symbol for Meg of her own failure to fit in: the twins' normalness tells Meg that she's not a misfit because she's a Murry, but because she's Meg.