Yinnar toddler Kelly Chapman and her baby twin sisters Katelyn and Emily were diagnosed six months ago with the genetic condition glycogen storage disease, which affects their livers' abilities to release glycogen.

The girls must be fed every three to four hours to maintain healthy blood sugar levels and avoid slipping into comas.

The disease is so rare it affects only nine other Australians.

Mother Sue Chapman said the easiest way to explain the disease, also known as Forbes' or Cori's disease, was to liken her daughters' bodies to trees.

``They're like the trunk of a tree with no branches, so the carbon dioxide can't get out,'' Ms Chapman said.

``If the carbon dioxide can't get out the tree will die.

``Their livers are five times the size they should be, and they store glycogen but can't get it back out again, so they have to continually eat so they have sugar in their blood.''

Ms Chapman knew something was ``not right'' when Kelly, now two, was a baby, but she was not diagnosed with the disease until three months after the twins were born.

Because the disease weakens the immune system, Kelly had become dangerously ill with tonsilitis and chicken pox, and began convulsing.

The girls' list of ailments is long and heartbreaking, with each symptom causing further health problems.

All three have enlarged livers and spleens, muscle wastage and cramps, and frequent nose bleeds because their spleens store too much blood.

They have distended abdomens, find solid foods difficult to digest and often vomit after eating.

``Liver replacements will probably be on the cards for all of them,'' she said.

``But because it's genetic their bodies could mutate the new liver to the condition of the previous one.

``There's not much sleeping because Katelyn has to be fed every three hours and she's got to have her blood sugar level tested one hour before to make sure her levels are still okay.