These brothers own the infamous Wallace store that sells alcohol to kids and hosts a dancing room for African-American teens (2.62). But they don't exactly do it out of the goodness of their hearts. Because it's the only store in the area, it is pretty much a "company store." The sharecroppers are indebted to the landowners and can only get credit here to purchase the things they need. Selling alcohol and providing a dancing room just means that they get the families even more in debt.
There's not much differentiation among these brothers: they're more of an ominous collective than individual well-developed characters. They represent the class of corrupt whites of the South during this time. And they're really bad news. Here's what Mrs. Lanier and Mr. Avery say about the Wallaces' brutal exploits:
"But them men dragged him [John Henry] and Beacon both outa that house, and when old man Berry tried to stop it, they lit him afire with them boys.
"It's sho' a shame, all right," said TJ's father, a frail, sickly man with a hacking cough. "These folks gettin' so bad in here. Heard tell they lynched a boy a few days ago in Crosston." (2.56-7)
We can be pretty sure that the Wallaces are "night men": not only do we later find out that they're responsible for burning the Berrys, but they also attacked Papa and Mr. Morrison on their way back from Vicksburg. Basically, they Wallaces act as self-appointed thug "enforcers" for the landowners' wishes. (We're almost certainly meant to think of the Ku Klux Klan when Taylor mentions the "night men.")
Are they racist caricatures? Maybe. Unfortunately, we think it's a lot more likely that they're a faithful representation of what some people really were like.