Emily, three, and Poppy-Mae, 22 months, have an incurable condition genetic disorder that causes a build-up of the amino-acid cystine in the cells of the body.
Only 2,000 people in the world have been diagnosed with the devastating disease, cystinosis.
Over time cystine crystallises in the cells, forming in the kidneys, the eyes, the thyroid gland and the liver.
Without treatment, sufferers are likely to experience complete kidney failure by about age ten.
Mum Jessica Kemp, 25, from Leeds, who is raising money for the Cystinosis Foundation UK, said: "Basically their bodies are slowly being taken over by the crystals which build up in their cells.
"Children with cystinosis don't have the best life expectancies but at least they each know what the other one is going through and they will always have someone around to look after them.
"When Poppy-Mae was also diagnosed it was devastating, but now I'm glad in a way that they have each other for comfort. They support each other in all sorts of little ways.
"Poppy is a very cuddly child and whenever Emily is feeling down or retching from her medications she will go to her and give her a hug.
"And when they have hospital appointments, Emily always makes sure Poppy has a toy to play with in the waiting room to take her mind off it."
The family discovered Emily was ill less than a year after she was born. Previously a perfectly healthy baby, Emily suddenly began vomiting frequently and losing weight.
Doctors were convinced she had picked up a passing virus, but Jessica's instincts told her differently.
Doctors at Leeds General Infirmary began testing for everything from leukaemia to cystic fibrosis. Emily was finally diagnosed with cystinosis.
She would need medication for the rest of her life, a feeding tube through her stomach to give her nutrition at night and in the future would require a kidney transplant.
Poppy-Mae's diagnosis was more straightforward as doctors already knew what to look for. But that did not make the news any easier to bear.
"When we found out that Poppy had it too, it was really hard. I was suffering from post-natal depression and the diagnosis just sank me into despair," said Jessica.
"But it was the girls who picked me up from it. They both cope really well with their conditions and their courage has inspired me."
Today Jessica and partner Darren juggle the demands of being the girls' full-time carers, which means making sure they take their medication every six hours.
The girls also need eye drops as the build-up of the crystals in their corneas makes them very sensitive to the light. Without them they would slowly go blind as the crystals developed.
The disease also affects their ability to swallow so both children are fed through gastrostomy tubes fitted directly into their stomachs overnight.