The distressed boy, 16, made two appearances via video link in Perth Children's Court on Wednesday, the day after the alleged attack in Perth's southeast.
"On the 21st July in Bentley it said you murdered (your mother)," the magistrate said during his first court appearance.
"Why would I do that? I didn't do that," the boy said, clearly upset.
"There is a further charge that you attempted to unlawfully kill (your sister)," the magistrate said.
The court appearance, in which pleas were not entered, followed the discovery of a 35-year-old woman's body at a home in the suburb of Bentley on Tuesday morning.
A girl, 12, fled the family home with several stab wounds.
Her brother, who cannot be named for legal reasons, initially refused to face the court for his second appearance.
He was eventually coaxed to sit in front of the camera at the Banksia Hill Juvenile Detention Centre, where he has been remanded in custody.
A bail application was not made on behalf of the boy, and he walked away from the camera part-way through the hearing.
Judge Wendy Hughes said the boy was now in the care of the Department of Communities.
Legal Aid lawyer Claire Rossi told the court an urgent application had been made to contract the services of a senior lawyer to represent the boy for his criminal charges.
The court heard a second lawyer would be needed to represent him for the child protection proceedings.
A mental health assessment and a communities department report were ordered before his next court appearance in two weeks.
Judge Hughes asked the detention centre staff to explain the proceedings to the boy, saying it could be confusing for him.
The boy's sister fled to a nearby highway for help after she was allegedly stabbed.
Passers-by provided her with life-saving first aid.
She was taken by ambulance to Perth Children's Hospital, where she remains in a stable condition.
WA Police Commissioner Col Blanch said the girl was receiving care for "very serious injuries".
"She is being well looked after ... surrounded by family.
Mr Blanch said investigators were treating the incident as "a family violence homicide against family members".
The boy, who was known to police, remains in custody
Western Australian Premier Roger Cook said it was troubling incident.
"It'll be distressing for the families and friends involved, for the people who lived in that community, and for the first responders," he told reporters.
"Our hearts go out to everyone that's been touched by this horrible incident.
The teen is scheduled to reappear in the same court on August 5.
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