It is clear that Jack still looks up to his big brother Atticus. Atticus helped put Jack through medical school, and Jack appears to have not forgotten. Jack picks up the Christmas presents for Scout and Jem, air rifles which apparently could not be purchased in Maycomb, and only half-truthfully tells the children that "Atticus wasn't interested in guns."
"You'll have to teach 'em to shoot," said Uncle Jack.
"That's your job," said Atticus... (Chapter 9)
Following Atticus's wishes, Jack gladly "instructed us on the rudiments thereof." Like Atticus, Jack has old fashioned values; like Atticus, who instructs Scout that it is not proper to use "common" words like "nigger," Jacks lectures her on the unladylike habit of cursing. Jack later punishes Scout unfairly for her fight with Cousin Francis, but he keeps his word not to tell Atticus the real reason for it. Jack goes to Atticus for advice afterward, telling his brother that
"I shall never marry, Atticus... I might have children." (Chapter 9)
Jack already realizes that things will be bad for Atticus once the Tom Robinson trial begins, and he agrees with his brother's assessment of the Ewells.
...he remembered them. He described them to Atticus, but Atticus said, "You're a generation off. The present ones are the same, though." (Chapter 9)
And like his brother, Jack can't resist poking fun at his old friend, Miss Maudie, who he jokingly asks to marry him each time he visits Maycomb.