They were the identical sisters who made a childhood pact to cut themselves off from the outside world and only talk to each other.

Known as the Silent Twins, June and Jennifer Gibbons devised a secret language which set them apart from friends, family, teachers and classmates.

It was a relationship that brought them to the depths of despair – they loved and loathed each other with equal passion – and eventually proved their downfall.

After a five-week spree of vandalism and arson, in 1982 they became the youngest patients at Britain’s high-security psychiatric hospital Broadmoor.

Silent Twins and a deadly pact: June Gibbons, pictured above, was locked in a bizarre and secret world with her sister Jennifer, with the two girls refusing to speak to anyone apart from each other. They were detained in Broadmoor for 11 years after a spree of violence but within hours of their release, Jennifer died in what biographer Marjorie Wallace suggested was a deadly agreement to allow her sister June to survive

Secret world: The twin sisters were alienated from the world around them, largely due to the pact they kept with each other, but also because of their nomadic childhood and the bullying they experienced for their skin colour in the predominantly white town of Haverfordwest in Wales. Pictured, the Gibbons twins as young children

Deadly pact: Biographer Marjorie Wallace claimed that Jennifer Gibbons had told her that one of the twins needed to die for the other to survive. Days later, Jennifer, pictured below, died and June, pictured above, later described her sister's death as tragic but also as a release

Detained indefinitely, the two teenagers were locked up alongside convicts such as Yorkshire Ripper Peter Sutcliffe and gangsters Ronnie and Reggie Kray.

Finally, after 11 gruelling years, they were given the news they had prayed for – they were being transferred to a lower-security unit in their homeland Wales.

But their release came at a tragic price: within hours of their release Jennifer had dropped dead of a sudden and lethal inflammation of the heart.

They should never have been locked up in Broadmoor. I know they did wrong but they didn’t kill anyone. It totally ruined their lives. Greta Gibbons

Now, 23 years after that fatal day, their older sister Greta has broken her silence to reveal the terrible toll the twins’ incarceration left on her family.

‘They should never have been held in Broadmoor,’ she told MailOnline. ‘I know they did wrong but they didn’t kill anyone. It totally ruined their lives.

‘Jenny should never have died – she was only 29 years old and should not have been discharged if she was not fit enough. She should have been in hospital.

‘And June could have had a much better life. She has never married or had children or fulfilled her ambition to be a writer.

‘If it had been me, I would have sued Broadmoor. I would not have let them get away with what they did.

‘But it was my parents’ choice and they always said that it would not bring Jenny back.’

Since their sister’s death, Greta, 58, brother David, 56, and sister Rosie, 49, have rallied around their parents Gloria and Aubrey and surviving twin sister.

Known as the Silent Twins, June and Jennifer Gibbons devised a secret language which set them apart from friends, family, teachers and classmates.

Deadly relationship: Jennifer (left) and June (right) Gibbons made a pact to keep themselves separate from the rest of the world, creating a secret language and mirroring each other's movements. They are pictured above with Marjorie Wallace, who wrote a book about their lives

New life: The twins were detained in the high-security psychiatric hospital Broadmoor for 11 gruelling years. But then they got the news that they were to be moved a lower-security unit in their homeland Wales. Pictured, Milford Haven where June Gibbons now lives quietly

Tragedy: On the day that they walked out of Broadmoor, just moments after the gate slammed shut, Jennifer collapsed on June - killed by a sudden, unexplained inflammation of the heart. Pictured, Milford Haven

While Gloria still lives a mile from the former family home in the market town of Haverfordwest, Aubrey is in a nursing home with vascular dementia.

Rosie has moved to London while the remainder of the family has decamped to the coastal town Milford Haven, eight miles away.

June, who dreamed of marriage, children and becoming a writer, has not fulfilled any of her ambitions.

Instead she lives in a rented house in the centre of the town and rarely goes out apart from visiting her family and meeting up with them on special occasions.

It went too far and although we longed to be normal we couldn't break out. We tried to get back to the outside world but it was too late. June Gibbons

‘June has had lots of offers to go on shows,’ adds Greta, a mother of two and grandmother of three, who works as an auxiliary nurse.

‘But she is too quiet for that. She doesn’t go out much. She used to work in the Barnardo’s charity shop but she now cleans for my Mum on Mondays.

‘We do have some fun. We went to Scotland last August for five days to visit the Edinburgh Tattoo and she really enjoyed that.’

It is 53 years on Monday since June and Jennifer Gibbons were born at an RAF hospital in Aden, in the Middle East, where Aubrey was stationed.

That Christmas he and wife Gloria, who met and married in Barbados, were posted to Linton-on-Ouse, in Yorkshire, with their four children.

The two ‘twinnies’- as their mother called them - barely spoke but the arrival of younger sister Rosie, in 1968, meant she had little time to worry about them.

It was only later that it emerged that the girls weren’t shy, or backward, but had made a decision not to communicate.

‘We made a pact,’ June explained in an interview after her sister’s death. ‘We said we weren't going to speak to anybody.’

Playing: Although June later confessed that their silence and secret language started as a game, she said that by the time they realised they were trapped they were unable to 'break out'. Pictured, Jennifer (left) and June (right) with author and chief executive of SANE, Majorie Wallace, in 1993

A nomadic childhood increased the twins’ alienation – their father was transferred to Devon, in 1971, and Pembrokeshire in 1974.

They were teased mercilessly about their silence and taunted about their skin colour at both primary and secondary school.

In fact the school bullying in the predominately white town of Haverfordwest was so severe that they had to be dismissed five minutes early to leave alone.

Gradually the twins retreated into their own private world, locking themselves into their bedroom and playing with dolls.

Juvenile delinquents get two years in prison. We got 12 years of hell because we didn't speak. We lost hope, really. We were trapped. June Gibbons

They chatted away in their own secret language – later discovered to be speeded up English – and synchronised their movements.

‘It started as a game,’ June explained later. ‘But the longer it went on the more trapped we felt.

‘It went too far and although we longed to be normal we couldn't break out. We tried to get back to the outside world but it was too late. We were twins but our personalities clashed.’

The twins left school at 16 and signed on the dole. That Christmas, Gloria gave them each a red leather-bound diary with a lock and key.

Immediately they began chronicling their thoughts. ‘Nobody suffers the way I do, not with a sister; with a husband, yes; with a wife, yes; with a child, yes, but this sister of mine, a dark shadow robbing me of sunlight, is my one and only torment,’ wrote June.

Within weeks they had both begun novels: June’s Pepsi Cola Addict, about a teen seduced by his teacher, while Jennifer’s Discomania about ‘insane violence’ at a disco.

But it did not relieve their isolation. At the age of 18, they lost their virginity to two American boys, who introduced them to drugs and alcohol.

‘We tried to be like normal teenagers,’ June says afterwards. ‘We were in love with them. We felt special with them.’

But the boys abandoned them. Soon they progressed to petty crime, taunting the police until they were caught and charged with 16 counts of burglary, theft and arson.

Chronicles: At 16, they were each given a diary. Immediately, each began an account of the desperate love-hate relationship they had. June described her sister as her 'one and only torment'. Pictured, Milford Haven

Disturbed: Once they had arrived at Broadmoor, the hospital psychologists deemed them deeply disturbed. They were surrounded by killers and rapists such as the Yorkshire Ripper Peter Sutcliffe and gangsters Ronnie and Reggie Kray. Pictured, Milford Haven high street

Shattered: For the family of June and Jennifer, there is little peace. They have rallied round the surviving twin, but they must live with the knowledge that the women's lives were ruined by the decision to lock them away. Pictured, Milford Haven high street

On the advice of their lawyers, they pleaded guilty at Swansea Crown Court, in May 1982, and were ordered to be detained at Broadmoor indefinitely.

There psychologists deemed them deeply disturbed as they took turns eating and starving, housed in the same cell and on opposite ends of the hospital.

‘Juvenile delinquents get two years in prison,’ June said later. ‘We got 12 years of hell because we didn't speak.

Her speech was slurring. She was tired and said she was dying. Then she slept with her head on my lap but with her eyes open. June Gibbons

‘We lost hope, really. I wrote a letter to the Queen, asking her to get us out. But we were trapped.’

In fact the girls felt so trapped that, according to their biographer Marjorie Wallace, they decided that the only way that they could survive was for one of them to die.

Ms Wallace revealed that Jennifer told her: 'I'm going to have to die'.

'I sort of laughed. I sort of said what? Don't be silly. You're 31 years old. You know, you're just about to be freed from Broadmoor. Why are you going to have to die? You're not ill. And she said, because we've decided.

'At that point, I got very, very frightened because I could see that they meant it. And then they said, we have made a pact. Jennifer has got to die because they said the day that they left Broadmoor, the day that they were free from the secure hospital, one of them would have to give up their life to really enable the other one to be free.'

In retrospect their deal proved prescient.

Finally, in 1993, doctors agreed to transfer them to Caswell Clinic, in Bridgend. But upon arrival Jennifer was unresponsive. She was rushed to hospital and pronounced death of acute myocarditis.

At the inquest, June said that her sister had been unwell the day before and during the journey.

‘Her speech was slurring,’ she added. ‘She was tired and said she was dying. Then she slept with her head on my lap but with her eyes open.’

Jennifer is now buried under a headstone engraved with a poem written by June. It reads: ‘We once were two/We two made one/We no more two/Through life be one/Rest in peace.’

But there is little peace for the rest of her family. In her book The Silent Twins, author Marjorie Wallace wrote: ‘They were failed the help they so desperately needed.

‘Instead they were locked away in a high-security hospital for 12 years where they met mainly killers, rapists and severely disturbed patients just at an age when they should have been stepping out into the world as the talented young women they might have been.’