Shawn Scott, 24, is the brother of 19-year-old Akein Scott, who was arrested on Wednesday night in the shooting, police Chief Ronal Serpas said.
Both face 20 counts of attempted second-degree murder, and Serpas said investigators believe the Scotts worked together in the shooting.
The mass shooting showed how far the city has to go to shake a persistent culture of violence that belies New Orleans' festive image.
Police suspect the brothers were involved in a neighbourhood gang and "engaged in a criminal lifestyle by choice," Serpas said.
Akein Scott appeared before a magistrate judge on Thursday. His bond was set at 10 million dollars.
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." Besides 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 the 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 dollars, 500,000 dollars 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 on 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.