A Miami-Dade County grand jury indicted 15-year-old Marc Wabafiyebazu on first-degree felony murder charges Wednesday.
Wabafiyebazu's defence lawyer, Curt Obront, said his client will plead not guilty.
"We have confidence in the criminal justice system and look forward to moving forward through judicial process," Obront said.
Wabafiyebazu was arrested March 30 after the shooting deaths of his 17-year-old brother, Jean, and 17-year-old Joshua Wright. The indictment returned Wednesday charges that Wright and Jean Wabafiyebazu shot each other.
The grand jury also indicted two other young men involved in the alleged drug deal on felony murder charges.
Media reports say Wabafiyebazu, who is being held in juvenile detention, will now be moved to an adult jail to await trial.
Wabafiyebazu's lawyer has said his client — whose mother is Roxanne Dube, a longtime Canadian diplomat who recently became general consul in Miami — plans to plead not guilty to any charges in the case.
Under Florida law, all suspects involved in committing a crime can be charged with murder if it leads to a killing.
Police say the Wabafiyebazu brothers were involved in an alleged marijuana drug deal gone wrong.
Investigators allege the brothers had gone to a home with the intention of stealing marijuana from a dealer. While the younger sibling waited with the car outside, the older teen went inside.
Several shots were fired inside the home, police allege, and Marc Wabafiyebazu rushed into the house but exactly what he did after that is not clear.
Officers who arrived at the scene found the older Wabafiyebazu and Wright dead inside.
Two other men, including Anthony Rodriguez, 19, the alleged dealer, were wounded and needed hospital treatment.
According to police reports, the Wabafiyebazu brothers — who had only recently relocated from their father's home in Ottawa to South Florida to be with their mother — took her personal vehicle with its diplomatic plates to the alleged drug meeting.
Police documents state Wabafiyebazu said he and his brother had pulled off similar drug-related rip-offs in Canada.
With files from The Associated Press.