Both sisters are now recovering from a double mastectomy - but unlike Mandi who had the debilitating surgery as part of her treatment for an aggressive form of breast cancer, Melissa underwent it as a preventative measure.

"There was a 97 per cent chance that I would develop the same cancer in the next five years," Melissa said. "So I'm the lucky one but I don't feel lucky - I feel guilty because Mandi's diagnosis has saved my life but now she's fighting for hers."

The 43-year-old women have led fairly identical lives. Both live in the northern suburbs with their husbands and have three children apiece. They share the same profession as nurses, the same zest for life and same community spirit.

They've always been inseparable and when Mandi was diagnosed with breast cancer in June 2014, Melissa took 12 months off work to be her carer and has since extended that.

"I initially felt a lump on my left side and had an ache in my armpit," Mandi said. "I thought the ache came from leaning down to pat my daughter in her cot but tests came back positive for breast cancer.

"It was diagnosed as stage 3 triple negative breast cancer - an aggressive form that is caused by a faulty gene."

Not the BRCA gene mutation which was highlighted when actress Angelina Jolie had preventative surgery, but a faulty gene that has yet to be identified.

Mandi started out with high hopes she could beat the cancer. However after four rounds of chemotherapy, four surgeries including the life-changing mastectomy, and six weeks of straight radiation, she's received devastating news.

Five more tumours have been discovered in her breast and back, and the cancer has spread to her skin.

"The only thing left for me to do is to go to Melbourne's specialist cancer centre, the Peter McCallum centre, and hope to get on a trial medication that will give me more time," she said.

"I also hope to be able to send tissue samples over to the US for potentially lifesaving testing to see if they can target the gene causing the mutation and then advise me on the best targeted therapy."

The family also want the gene identified to better protect their own children.

"It's so important to know your family history, we wished we'd known more about ours," Melissa said. "We knew our grandmother had breast cancer but she was diagnosed in her 70s and lived to 95. We have also found out that her sister had breast cancer, and now her daughter - our aunt - has been diagnosed too.

"We need to know more about this gene so we know what our children's chances are of getting breast cancer and how they can protect against it."

Mandi's family members have set up an online fundraising campaign to raise the funds for her to continue to fight.

The gofundme campaign raised nearly $5500 of the $20,000 goal in just five days.

"We've always done everything together, we're very close and know what the other is thinking without having to speak," Melissa said.

"But we can't do this on our own - it's expensive being sick. Our family, friends and community have been fantastic and we hope we can get even more support for Mandi."

To help, visit gofundme.com/mmfbc.