Lauren Smith, 23, was rushed to hospital when her waters broke on Christmas Day last year.
She and partner Graham Lucas, 27, had been told their twins had the life-threatening twin-twin transfusion syndrome, and girls Elouise and Ava were born prematurely at 25 weeks in January 2016.
Both twins were put on life support, but the family was advised to turn off Elouise’s machine at 18 hours.
Elouise had weighed just 400g, less than a bag of sugar.
Four weeks later doctors advised the family to switch off the life support machine of twin sister Ava, but her parents refused and she kept on fighting.
She remained on life support for 10 weeks, underwent numerous lifesaving treatments during her first four months, was treated at five different hospitals and was often in a critical condition.
Ms Smith said: “It was the most traumatic time of my life when the girls were born.
“I didn’t see them for hours and when I did, it was clear they weren’t ready to take on the challenge they needed to face.
“My babies were attached to machines and wires which completely covered them - I ran back out of the neonatal intensive care unit crying in fear and shock. It was terrifying.”
She said she felt things would get worse, and they did.
“Within hours we were told Elouise was dying and we had to make the hardest decision - a decision no parent should have to make - we had to turn off our baby’s machine,” she said.
“We were broken-hearted and inconsolable that one of our precious babies had died but we had to stay strong for Ava and pray she would pull through.
“During those first few weeks her little body was put through so much, it was unbearable to watch.
“Our heartache continued when Ava was four weeks old and we were advised to turn off her machine.
“We had already lost Elouise and couldn’t lose Ava too.”
The family, from Peterborough, went to specialist hospitals including London’s Queen Charlotte’s and Chelsea Hospital and The Rosie, in Cambridge.
They were able to be at Ava’s bedside thanks to support from The Sick Children’s Trust, which provides accommodation for families.
After four months in hospital, Ava was well enough to go home and has continued to amaze her parents and medical team with her progress.
Ava has now been at home for seven months and is looking forward to Santa’s visit on Christmas Day.
“Ava never stopped fighting - she was put through a lot but never gave up and we are so proud of her for that,” said Ms Smith, who is an ambassador for the Sick Children’s Trust.
“This Christmas will be very difficult as even though we are celebrating Ava being alive, we will be remembering our beloved Elouise.
“We will lay flowers in our garden for her and think of her all day.
“We know she will be looking down on Ava as we make the day one to remember.”
Abi Abdel-aal of the Sick Children’s Trust said the charity was glad to help the family during the “most traumatic time imaginable”.