Lily Thomas was just nine years old when she saved her sister Ava's life.
Little Ava has MPS 1 Hurler syndrome, a rare genetic condition, and needed an urgent bone marrow transplant.
The girls appeared on Today Extra with their parents on Thursday, where their mother Lina described the transplant as a 'miracle'.
'It was definitely a miracle,' Mrs Thomas said.
'We just didn't expect it, we tested all the kids - we got the two boys as well - and for Lily to be a match was just amazing.'
Ava was only five months old when she was diagnosed with MPS 1 Hurler syndrome.
People with the condition lack an enzyme that helps break down long chains of sugar molecules called glycosaminoglycans, according to Medline Plus.
Without the enzyme, glycosaminoglycans build up and damage organs, including the heart.
Mrs Thomas said Ava was born premature and had whopping cough when she was four weeks old.
When she still was not thriving and doctors performed a metabolic urine test.
It was only then that they discovered she had the condition.
She underwent enzyme replacement therapy, before they were able to perform the bone marrow transplant.
'I just was very happy I could help her,' Lily said.
The surgery has helped Ava to reach her milestones, but she still has some complications.
'She's still got a fair bit to go with all the skeletal issues, but she's doing well,' Mrs Thomas said.