Police suspect the brothers were involved in a neighborhood gang and "engaged in a criminal lifestyle by choice," Serpas said.
Akein Scott appeared before a magistrate judge Thursday. His bond was set at $10 million.
At a news conference at the shooting site near the French Quarter, Mayor Mitch Landrieu accused the suspects of "callously shooting into crowds of hundreds of citizens."
"The culture of violence is unnatural and unacceptable," Landrieu said, calling the shooting site "holy ground."
In addition to the Scotts, Serpas said police arrested four people who are accused of helping Akein Scott elude capture. Serpas said their charges are being accessories after the fact to attempted second-degree murder and obstruction of justice. They range in age from 19 to 32, he said.
At Thursday's court appearance, Akein Scott, shackled and wearing an orange prison jumpsuit, stood silently as his court-appointed attorney handled the proceedings.
Prosecutors said a witness picked out a photo of Akein Scott, 19, from a lineup. An arrest affidavit said the unidentified witness told investigators that Akein Scott was the person seen in a surveillance video that police released to the public as they searched for him for three days. The witness also said Akein Scott was carrying a silver and black semi-automatic handgun at the shooting scene, according to the affidavit.
Magistrate Judge Gerard Hansen set Akein Scott's bond at $10 million $500,000 on each of the 20 counts in the Mother's Day shooting case. Authorities earlier said 19 were wounded, but prosecutors told Hansen the number had increased.
Akein Scott was arrested Wednesday night in the Little Woods section of New Orleans. He already faced gun and drug possession charges and was out on bond at the time of Sunday's shooting.
At a later appearance Thursday morning, a state District Court judge ordered Scott held without bond pending additional hearings in that case. Akein Scott faces a felony charge of illegally carrying a weapon while in possession of a controlled dangerous substance.
In the neighborhood where the gunfire shattered the festive parade known as a second line, residents awoke Thursday to the news that the manhunt for Akein Scott apparently had ended. Police had been searching for Akein Scott since identifying him as a suspect Monday from the surveillance video.
Courtney Moles, whose apartment overlooks the shooting site, said she didn't feel her safety was in jeopardy while police searched the city.
"I didn't really think he would come back. It's more personal than that," she said. "He wasn't going to that second line to make national news. He was probably settling some kind of score."
Moles, 24, said the arrest underscores the city's crisis of violence among its young people. "His life is over now, too," she said of Scott.
Police have not established a motive. Officials initially said three people were spotted running away from the shooting scene, but Akein Scott had been the only suspect identified publicly until his brother's arrest was announced Thursday.
Investigators launched an intense search for Akein Scott, with police Superintendent Ronal Serpas urging him to surrender at a news conference Monday and warning the teen that "we know more about you than you think we know." At one point, SWAT team members and U.S. marshals served a search warrant at one location but did not find Scott.
Police offered a $10,000 reward in the case, and investigators received several tips after images from the surveillance camera were released.
The video released Monday showed a crowd gathered for the parade suddenly scattering in all directions, with some falling to the ground. They appear to be running from a man in a white T-shirt and dark pants who turns and runs out of the picture.
As many as 400 people had come out for the event. Officers were interspersed with the marchers, which is routine for such events. The crime scene was about less than two miles from the heart of the city's French Quarter.
Two children were among those wounded.
The mass shooting showed again how far the city has to go to shake a persistent culture of violence that belies New Orleans' festive image.
Gun violence has flared at two other city celebrations this year. Five people were wounded in a drive-by shooting in January after a Martin Luther King Jr. Day parade, and four were wounded in a shooting after an argument in the French Quarter in the days leading up to Mardi Gras. Two teens were arrested in connection with the MLK Day shootings; three men were arrested and charged in the Mardi Gras shootings.
Associated Press writer Michael Kunzelman contributed to this report.