We are given this information in the very first chapter, that describes the family history of the Finches and also details how Atticus and his siblings made their way in the world. Also, the closeness of the family is indicated, as Atticus supports his younger brother to follow his chosen career path until he is able to establish himself, showing sacrificial love and how the Finch family supported one another.
Note what we are told about John Hale Finch, the brother of Atticus:
During his first five years in Maycomb, Atticus practised economy more than anything; for several years thereafter he invested his earnings in his brother's education. John Hale Finch was ten years younger than my father, and chose to study medicine at a time when cotton was not worth growing; but after getting Uncle Jack started, Atticus derived a reasonable income from the law.
Note the details given of how Atticus was forced to practice "economy" through the amount of money he needed to give to his younger brother to support him in his study. It is only after he has finished his study that Atticus is able to earn a "reasonable income" from his profession.
As was mentioned in the previous post, John Hale "Jack" Finch is a doctor in the novel To Kill a Mockingbird. At the beginning of the novel, Scout explains her family's background and introduces the audience to her uncle, Jack. Uncle Jack is ten years younger than Atticus and ended up leaving Finch's Landing to go to Boston to study medicine. Scout mentions that Atticus paid for his brother's education when he became a lawyer. Throughout the novel, Uncle Jack plays a minor role and is a rather pleasant, amusing character. He continually jokes with Miss Maudie and even buys Jem and Scout air rifles for Christmas. After Scout gets into a fight with her cousin, Francis Hancock, Uncle Jack bandages her wounds. Although he has no children of his own, Uncle Jack gets along well with Jem and Scout.