Why was Ponyboy proud of Soda and Darry?
In addition to the court scene, there are other times in the book that Ponyboy expresses his pride in his brothers.
In Chapter 2 when Ponyboy first meets Cherry at the movies she comments that she thinks Soda is "a doll." Pony's response to that is "I grinned with pride...it's not every day I hear Socs telling me they think my brother is a doll."
More than once Ponyboy mentions Darry's work ethic. In Ch 7 he says, " Darry worked on two jobs at once and made good at both of them, and about his outstanding record at school." Once Ponyboy realizes he was mistaken about Darry's coldness, it is evident to the reader that he is proud of his brother and just how much Darry gave up in order to keep the family together after the death of their parents.
On page 168 in my copy of the book, two pages in to Chapter 12, Pony is at his hearing. His two brothers, Soda and Darry, take their turns to be questioned by the judge.
The judge asks both of them if they were pretty good friends with Dallas Winston, who had been killed by the police not long before. Darry says yes, looking straight at the judge. Soda says yes too, but looks like he thinks he is hurting Pony's case.
Pony says then that he is proud of them. Presumably he is proud because they have told the truth and not denied being friends with Dally.