Before the attack, Mohamed Mire said, his family appealed unsuccessfully to local authorities as his 29-year-old brother, Muhaydin Mire, became more and more paranoid.
Unsure of what to do, Mohamed said he even booked a flight for his brother back to Somalia, where he as born, for Sunday — the day after the stabbing at Leytonstone Underground Station.
Both victims were treated for non-life threatening injuries.
The revelations come as Muhaydin was charged with attempted murder and as British authorities investigated the attack as a terrorist "incident." Witnesses told Channel 4 that they heard Muhaydin shout "This is for Syria" after being taken into police custody.
On Monday, the United Kingdom remained under a "severe" threat from international terrorism.
Mohamed Mire told the channel that Muhaydin — who has lived in Britain since he was 12 — was diagnosed with paranoia. In 2007, he was hospitalized for three months, but marijuana use, Mohamed said, only exacerbated his mental illness.
"He's seeing demons and all that," Mohamed said, adding that his family tried to have Muhaydin committed.
After talking with his mother in Somalia, Mohamed said, he booked a flight for his brother.
"As far as I know, he wanted to go," Mohamed said.