The boy is charged with manslaughter and faces other charges related to an alleged armed robbery a few days before the death of his 16-year-old brother.
The identity of the accused and the victim cannot be revealed because they are minors.
The Crown had opposed bail and, on Thursday, Quebec youth court Judge Dominique Wilhelmy agreed.
She ruled the accused, who is now 13, still presents a danger to society.
Wilhelmy noted he consumed marijuana and alcohol, took his mother's car on the sly and proved difficult to control.
''This is a young man well installed on the road of delinquent activities,'' she said.
The judge noted in her ruling that the mother had denied her son had any substance abuse problems; denied he had taken her car without her knowledge; and didn't believe he'd committed a robbery.
She dismissed the notion of sending the boy to live with his mother and grandmother, saying they would not be able to exercise the necessary parental authority to look after him.
"The judge decided they were really willing and it's a sad situation, but in light of the accused's own behaviour, they wouldn't be able to control him or ensure the protection of the public," said Crown prosecutor Marie-Claude Bourassa.
"I won't go into the details, but she (the judge) also considered his own behaviour outside two very serious accusations, the main ones — the robbery and the homicide," Bourassa said.
The boy was charged with manslaughter after his brother's death on Jan. 21 at the family home in Dorval, a suburb in western Montreal.
A robbery charge was added following his arrest.
Two trials will be held to deal with the charges.
The accused pleaded not guilty to the charges during his initial appearance.
He faces a maximum of three years in detention if found guilty.
The accused returns to court next Wednesday when a date will be set for the resumption of the proceedings. The case is expected to be heard in the coming months and there will be no preliminary hearing.