Barbara Stone, 36, disclosed yesterday that as someone who respected the law, she had accepted the possibility of his guilt when police arrested her brother last year. But her doubts mounted as the initial "evidence" against him fell away.
They were compounded this week when a former prisoner, Barry Thompson, retracted what he had said against Stone in court.
"I don't want to rub it into the Russells that it's a miscarriage of justice. That's not fair - it's too soon. But unless they prove to me different, I believe he's innocent," she said.
Stone was convicted on Friday of the murder two years ago in Kent of Dr Lin Russell, 45, and her daughter, Megan, six, and the attempted murder of Josie, now 11. But he always denied the crimes and has appealed for anyone who could help clear him to come forward.
Ms Stone said yesterday that a great deal of information had been forthcoming and it was hoped to launch an appeal within two weeks.
At the trial at Maidstone Crown Court, testimony from three convicted criminals was the key to the case against Stone. But his sister said nobody should be convicted on the uncorroborated evidence of criminals. "I don't feel they should convict and send him away for the rest of his life on the word of another criminal."
Ms Stone chose to speak to The Independent yesterday at a meeting arranged by the Zito Trust, the mental health pressure group which is working with her to highlight the issues raised by the case.
Stone, 38, of Gillingham, Kent, was a drug addict with a long criminal record and a psychopathic personality disorder who had asked to be admitted for in-patient hospital care in the weeks before the killings.
Despite his sister's initial willingness to believe in Stone's guilt, many initial lines of inquiry, such as forensic tests, had failed to link him to the crimes. "As the evidence fell away, a fresh approach should have been taken," she said.
Unlike most murder cases involving community care patients, Ms Stone said her brother had been receiving good care for his mental health problems at the time of the murders. But the system failed at the point when he needed a bed.
West Kent Health Authority has denied Stone asked for in-patient care before the killings, but his sister said he was "actively asking" for a bed and would have been inside at the time of the killings had he been given one. "He had on-going community care, but at the crucial moment it failed him. Part of his community care should have involved a bed being available."
She criticised the health authority for denying it had any responsibility for him, claiming he had an untreatable personality disorder and therefore lay outside the remit of the Mental Health Act.
Psychiatrists have, in recent years, interpreted the law as meaning a person can be detained only if their condition is regarded as "treatable" - a narrow attitude condemned this week by Jack Straw, the Home Secretary.
Ms Stone said that ironically, in her brother's case it was quite clear that they were treating him and were making a notable difference.
"For the first time, perhaps, they had built up some trust between himself and the medical services. He was willing to discuss everything and anything with them."
But she said the law needed to be changed so that beds were always available if needed and health officials were always obliged to help people with personality disorders.
Stone had once undergone assessment at Broadmoor after trouble in prison and had fantasised about killing in the months before the Russell murders. He was eventually admitted as an in-patient in Kent a month after the Russell killings. But, Ms Stone asked, if he was really dangerous, why was he not admitted sooner?
As an aside, she noted that although the authorities claimed he had never been a paranoid schizophrenic, a condition considered treatable under the Mental Health Act, "schizophrenia" was the reason given for his receipt of disability living allowance.
Ms Stone, who had spent the last three years doing a psychology degree, said her own children - Nikki, 15, and Nathan, 13, who is autistic - were devastated by the conviction. She added that Stone "was totally devoted to them. They don't believe he did it.
"They've always known he has had problems and they just referred to good days and bad days. But there was never a day bad enough when he hadn't got time for them."
She said that throughout her brother's life, people who could have helped him had failed to do so. As a child he had lived in numerous children's homes because of the turbulence of their own dysfunctional family life.
But they let him down, too. "They took him out of what they considered an unstable environment and put him into another home, and another one, and another one."
Ms Stone said: "He's paying the price for the failings in the systems - the health system, the lack of addiction facilities, the prison system, and social services. He's paying the price for them all. Now they're all on trial."
But she said she would continue to stick by him. "He's my brother, I love him and I will always be there for him."Reuse content