The emaciated sisters, perfectionists who studied biomedical science and physical education at university, are also compulsive long-distance runners.

They have wasted away for two decades, at times slipping to 28kg.

Though only 34, their bodies have started to break down.

They are shrinking and they have the bone structure of women aged 70 to 100.

But the siblings also have a history of criminal offences and drug use, believed to be linked to their psychological disorder.

Clare was this week sentenced to two months' jail for a series of thefts.

Geelong magistrate Ian von Einem said he saw no option but jail to stop Clare from self-destructing.

"It's just right out of any sort of control. She continues to want to injure herself," he said.

Mr von Einem said Clare's anorexia, drug addiction and continual lies were a tragic combination.

"Sadly and unfortunately, she is going to have to be treated within the prison system for a short term," he said.

But Clare, who threatened to completely stop eating if locked up, was given a temporary reprieve and released on bail while she appeals against the jail term.

Sister Rachel also faces criminal charges in the coming months for drug-driving, heroin use and an assault in which she allegedly pushed a victim on to train tracks.

The twins both developed severe eating disorders in their early teens.

They became addicted to running and trained constantly for marathons.

They kept running until both recently suffered stress fractures in their feet.

The twins are inseparable. In 1996, when Rachel was so ill she was admitted to a psychiatric unit at Royal Melbourne Hospital, Clare voluntarily admitted herself too.

Rachel last night told the Herald Sun of her sense of hopelessness and loss of self-esteem.

"No one understands the disease until they have lived it. It's like the Grim Reaper, a black hole in your soul," Rachel said.

"Our bodies have deteriorated to a level where they are damaged . . . it's a terrible disease, but it's not a choice.

"We don't feel like we've got anything much to give the world. We feel we are a burden.

"We don't feel we are special to anyone, except to our family."

Parents Bob and Moya told of the heartbreak of watching their daughters slowly die.

The couple check the twins' beds every night, fearing they will succumb to their disorder or die of an overdose.

Moya said her daughters, born eight weeks premature, were diagnosed anorexic at 14.

"They were girls who were good at everything. They did very well at school, were good at music and athletics," Moya said.

"They are intelligent and sensitive, but they don't like the world the way it is.

"I think the anorexia is their way of not wanting to cope with it."

A police source said the courts had tried to give the twins every opportunity to reform, but repeated offending and an inability to change left prison the final option.

"At the end of the day it's the only thing left. Everything has been tried," the source said.

"They didn't want to have to come to this, but it was actually the very last resort (for Clare)."

Previously, the twins have been directed to counsellors and specialists for their disorder and drug problems, without success.

"Many people have tried to help our girls, but they eventually get put in the too-hard basket," Moya said.

"My doctor said they don't know what to do.

"But you don't give up on your kids. You just keep going."