'I live in fear that I'm next': Woman, 23, faces terrifying wait to discover if she will fall victim to the same mystery genetic illness that killed both of her sisters

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A Sydney woman who watched both her sisters die after mysteriously contracting Tay-Sachs Disorder 20 years ago lives in constant fear of one day suffering the same fate.

Venessa Chidiac, 23, from Earlwood revealed to Daily Mail Australia the heartache her family endured after her siblings Mary and Belinda were struck down by the illness just a few weeks apart.

Doctors were never able to tell her family why they were afflicted by the genetic disease later in life when it is normally diagnosed soon after birth.

'Yes, of course, I have lived in fear always that it would happen to me one day, my parents especially, even if I had a headache I would go straight to the doctor,' she said.

Venessa Chidiac (pictured with a framed photo of her late sisters Mary and Belinda) was honoured for her charity work when named Miss International Liaison last year. The mystery of how her siblings had the onset of the genetic Tay-Sachs Disorder has left Venessa with unanswered questions and her own fears 

Venessa Chidiac, 23, threw herself into charity work after her sister Mary died in May last year after years battling a degenerative genetic disorder

The Chidiac sisters pictured together in the same outfits before Mary (left) and belinda (right) were diagnosed with Tay-Sachs disorder - Venessa (centre) has spoken of her family's heartache and the mystery behind how they were diagnosed with the genetic disease later in life

Tay-Sachs disease is a rare and fatal genetic disorder that destroys the nerve cells in the brain and spinal cord causing cognitive and motor skill deterioration.

It is caused by an enzyme deficiency which prevents the body shedding fatty waste in the brain. 

According to statistics from the Medical Journal of Australia, on average there are less than two Tay-Sachs related births each year.

Mary died at age 27 last May; Belinda was just 18 when she passed away eight years earlier.

Venessa says they were informed this late onset of the disorder was the only documented case of its kind in Australia.

Tay-Sachs occurs when children inherit the same gene from each parent - in a cruel twist Mary and Belinda were diagnosed within weeks of each other.

'They began by stuttering, then it was problems with their leg movement, no concentration and teachers told my parents "your children are acting odd".'

The girls, aged 6 and 8 at the time, were tested and the family got the shock of their lives in the mail.

Mary pictured in hospital during one of the Chidiac sisters' many admissions after being struck down with the symptoms of Tay-Sachs Disorder. 'They were in and out of hospital' said their young sister Venessa

Venessa Chidiac, 23, from Earlwood revealed the heartache that she and her parents Raymond (l) and Lametta (r) endured after her siblings Mary and Belinda were struck down by the illness just a few weeks apart

Belinda (left) and Mary Chidiac (right) seen at their family's Earlwood home before they were diagnosed with the disease in 1996

Belinda seen with school friends from Undercliffe Public School at Earlwood from her kindergarten class

'For them to be at school in primary and be diagnosed then was a real shock, they (doctors) literally gave a letter to mum to tell her they had just six to eight months to live,' she said.

But the siblings lived for much longer than expected and the workload was shared among the family which refused to place the girls in respite care.

'Belinda used to sleep with my mum and I would look after Mary, they needed 24 hour care never but my mother (Lametta) and father (Raymond) never sought out help.'

Despite the lingering doubts Venessa has established a charity called Give Them Hope and lats year was crowned the Touch of Goodness Miss International Liaison for her philanthropic work.

On May 11 she will join with her lifelong friend Vivien Challita in the second annual Walk of Hope from Earlwood to Berrima to raise awareness for carers.

The Chidiac family say they were told by doctors that Mary (left) and Belinda's (right) case was extremely rare

Venessa Chidiac (pictured with Mary in wheelchair) has spoken about the genetic illness which claimed her older sister's life

Venessa Chidiac pictured with her parents Raymond and Lametta after being honoured for her charity work in 2015 just weeks after the death of her sister Mary

The 130 kilometre walk will begin at the Earlwood Our Lady of Lourdes Catholic Church and finish at the Shrine of Our Lady of Mercy in Penrose Park at Berrima. 

'Whatever funds are raised will go directly towards families that are in need, whether that's to pay for medical bills for a week, a month, it will be given to the family to help them just that little bit,' Vivien said.

She said the money raised will go to those needing assistance with medication costs and professional help 'or just trying the get through the daily grind while being a support for a child or adult with a disability'.

'Viv has honestly been my biggest supporter from day one, never letting me fall back without pulling me forward instantly,' added Venessa.

'I'll be doing this walk in memory of my sisters and all those with a special need who struggle to do the simplest things in life that we take for granted.'

Belinda (l) and Mary (r) - the first signs they were becoming ill came when they both began stuttering and had problems with their leg movement. Their younger sister has shown no signs of the illness but has lived with the fear that she could one day be struck down by the same disorder