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When Betty Anne Waters's brother Kenny was convicted of murder, the then pub waitress and high school drop-out was so convinced of his innocence, that she did the impossible and trained to become a lawyer to overturn his sentence.

It was an 18 year struggle, and one which was to dominate her life.

Betty Anne and Kenny grew up together in the small town of Ayer in rural Massachusetts, and had what she describes as a 'wild' upbringing.

They had a largely absent single mother, and Kenny would often get into trouble with the police.

But when in 1980 a neighbour was brutally murdered and Kenny was charged with the offence, Betty Anne was convinced he was innocent.

On the night the murder was said to have been committed, Kenny had been working at the local diner, and the following morning he had appeared in court on an earlier charge of assault.

It was said Betty Anne, "the perfect alibi."

And so Betty Anne was shocked when, in 1983, her brother was found guilty and sentenced to life imprisonment without parole.

With very little money, and so convinced that Kenny would be freed, the family had not hired a specialist lawyer.

In prison, Kenny tried to commit suicide, and confided to his sister, "I'm just not going to make it here as an innocent man."

Betty Anne was so upset, and so sure of Kenny's innocence, that she promised her brother she would study to become a lawyer to personally represent him in court.

She said he had complete faith in her and said, "I don't care if it takes you 80 years, if you tell me you'll do it, you will, and I know you'll find a way to prove my innocence."

So, despite not having finished high school, the waitress and mother of two kept her promise and enrolled at the local community college.

She said, "I figured I would at least keep my brother alive just by being there."

It was a long struggle as she spent 12 years putting herself through college and law school, but she eventually qualified as a lawyer.

As Kenny's attorney, she successfully discovered new DNA evidence - a science that had not been available at the time of her brother's conviction.

With the help of the - a US group who help overturn wrongful convictions - she successfully had her brother exonerated, and in 2001, after 18 years in prison, Kenny was finally freed.

Betty Anne went on to spend a further seven years tracking down evidence that the police had knowingly sent the wrong man to prison, and in fighting a civil suit on behalf on her brother.

She won her case, and in 2009 the town of Ayer agreed to settle a lawsuit for Kenny's wrongful conviction.

In all, Betty Anne spent almost three decades dedicated to clearing her brother's name. She also lost her marriage in the process when her husband complained she loved her brother more than him.

And now the story of her incredible life has been turned into a Hollywood film called .

She has never practised as a lawyer again, though she volunteers for the Innocence Project, and continues to work in a Rhode Island pub.

Betty Anne Waters spoke to Outlook's Jo Fidgen about her extraordinary personal story of devotion and determination.