why would you think it would be important for Atticus to invest in his brother's education?

Jack was ten years younger than his brother, and because of this, he was in his own way Atticus' responsibility. Farming was no longer profitable, and Atticus wanted to insure his brother's future.

During his first five years in Maycomb, Atticus practiced 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.