In the story, the four Cowgill brothers are the sons of Mr. Cowgill, the dairy farmer. Although we don't know the names of the oldest three boys, we are provided the name of the youngest: Ernie Cowgill. Ernie is tall and big for his age, despite the fact that he was the "runt of the litter" when he was born.
At the time of the story, Ernie is almost sixteen, but he's still in the fourth grade. His three older brothers are bigger and taller than he is, and Grandma Dowdel tells Joey that Ernie's brothers are the "ones you wouldn't want to meet up with in a dark alley."
In the book, the Cowgill brothers are ne'er-do-wells; they're responsible for uprooting Effie Wilcox's privy, shooting up Grandma Dowdel's mailbox, and breaking into Grandma Dowdel's house. The Cowgill brothers are basically destructive bullies who prey on defenseless older ladies. However, they meet their match in Grandma Dowdel.
One evening, Grandma Dowdel lays in wait for the Cowgill brothers when they break into her house. She cherry-bombs the boys; when the smoke clears, the old lady holds the boys hostage with her shotgun until their parents show up. When Mr. and Mrs. Cowgill attempt to rationalize their sons' actions, Grandma Dowdel shows Mr. Cowgill the mouse in the milk bottle. She tells Mr. Cowgill that his customers will eventually keep their own cows again if he's not careful about sanitation.
Mr. Cowgill is dumbfounded at the suggestion that he could be responsible for the mouse, but Grandma Dowdel seals her case by proclaiming that "a bunch of worthless boys who'd ransack the town every night is apt to drop a mouse in the milk..." Mr. Cowgill, knowing that his business will suffer if the town ever gets wind of the affair, decides to give Grandma Dowdel her "justice." He has the boys line up by age and whips them with Grandpa Dowdel's old leather strop. Joey notes that the Cowgill brothers "squealed like stuck hogs" during their punishment. So, although the Cowgill brothers are physically intimidating for their age, they still lack discrimination and good judgment, as can be seen in this chapter.