Mohamed Mire said that his brother Muhaydin, who appeared in court Monday, suffered from paranoia and hallucinations made worse by drug use, but that he was judged as no threat to the public.
"Drugs influenced him, just cannabis," he told Channel 4 News.
"It gave him mental problem. He was diagnosed by doctors and treated in 2007 for paranoia. He was in hospital for three months in 2007."
After recovering from that bout, the suspect went on to work for taxi firm Uber but "then he got back into the same thing and went a bit crazy," said the brother.
"That started in August of this year. He started calling me up and saying odd things.
"Not radical, it's a bit like jumping around talking nonsense and sort of like talking saying he's seeing demons and stuff, people following him.
"We tried to call the local authority, they could not help him because they said he's no harm to people and he's no harm to himself.
"I talked to the police and they came and looked at him and that was 22 October."
Muhaydin Mire, 29, from east London, is accused of attempting to murder a 56-year-old man at Leytonstone station on Saturday night, shortly after he and his family had agreed for him to leave Britain.
"I decided to book a ticket for him on this Sunday. He was okay as far as I know," said the brother. "He wanted to go."
During Monday's brief hearing, Mire spoke only to confirm his name, date of birth and address and was remanded in custody until Friday when he will again appear at the Old Bailey in London.
Prosecutors allege that Mire punched his victim to the ground and repeatedly kicked him before taking hold of the victim's head and cutting a 12-centimetre wound in his neck.
The victim, referred to in court as Male A, was in surgery for five hours after the attack, the prosecution said.
Prosecutors also allege that images and flags associated with Islamic State jihadists were subsequently found on Mire's mobile phone.