Olivia and Isabella are identical twins but they don't look as alike as they used to.
Three cycles of intensive chemotherapy have left Olivia shorter than her sister and blind in one eye.
The treatment for leukaemia also caused her hair to fall out although it has now grown back in blond curls.
But findings from a unique study of the twin's blood could one day mean other children with leukaemia need not go through such punishing treatment.
Doctors have found "pre-leukaemic" stem cells in both girls which are the root cause of the illness.
Drugs to specifically target the stem cells could be more effective than current treatments by killing the leukaemia at the source, researchers believe.
It could also mean doctors could reduce the amount of chemotherapy some children need by stopping as soon as the stem cells have been eradicated.
Olivia is doing well and in remission, but Isabella still has a one in 10 chance of developing the disease because her bone marrow harbours the pre-leukaemic stem cells.
It is likely the risk will drop as she gets older but until then she must undergo regular checks to ensure she is cancer-free.
The twins' mum, Sarah, said Isabella still has a long way to go.
"They said the cells might die off naturally but obviously it is something which is hanging over us.
"It would be very hard to have to get through it twice but we are trying not to think about it too much."
Olivia (left) was only two and a half when she was diagnosed
She said Olivia had been fantastic through all the rounds of brutal treatment.
The chemotherapy, made her very ill, in pain and unable to walk.
And it also weakened her immune system, opening the way for an infection which damaged her eye.
"Olivia is a real toughie.
"She's such a strong person, she made it quite easy on us.
"But when you see one really healthy child and one that's not healthy, that's really hard to watch."
The twins turn five in February and have both now started school.
Sarah said they had always been very happy to contribute to the research.
"Anything that would make the treatment less invasive and less hard on the little ones would be fantastic."
"I just don't want to see any child go through that."
Olivia's illness has been a strain on the family with Sarah and her husband Justin suffering many sleepless nights.
Sarah said that the risk of Isabella developing leukaemia was always at the back of their minds but they try hard to keep positive.
"We are just thankful. We know we have been lucky compared to others."