John Prevost knew there was a chance his brother could be elected pope.

“Last Saturday when I was at church, one of the priests came over and told me the odds in Las Vegas were 18 to 1,” said Mr. Prevost, who lives in suburban Chicago. “He didn’t have a doubt. He thought it would definitely be my brother.”

But Cardinal Robert Francis Prevost, who was preparing for the conclave, shrugged it off when his older brother called from Illinois.

“He said, ‘No way, not going to happen,’” recalled Mr. Prevost, 71, who is retired from a career as an educator and school principal.

Of course, it did happen. Cardinal Prevost is now Pope Leo XIV, the first American pontiff. And for his friends and family back in Illinois, where the pope grew up, everything is different.

In a wide-ranging interview on Thursday afternoon at his home in New Lenox, a tidy city of 27,000 people about 40 miles southwest of downtown Chicago, John Prevost reflected on his brother’s ascent to the papacy, the new pope’s values and his American roots.

Leo, whom Mr. Prevost is accustomed to calling Rob, “has great, great desire to help the downtrodden and the disenfranchised, the people who are ignored,” Mr. Prevost said. He predicted that his brother would carry on the legacy of his predecessor, Pope Francis.