The condition - called auriculo-condylar syndrome - can cause life-threatening problems with breathing because the tongue is forced to the back of the throat.
The siblings' mother, Deryn, says Kayla is affected worse than Bodee.
Kayla would probably have died at birth if not for the special preparations made for her caesarean delivery at the old National Women's Hospital in Epsom.
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The condition was detected in pregnancy after having been picked up in Bodee as a young child.
"For Kayla's birth, we had 27 people in theatre," Mrs Nield says.
As soon as Kayla's head was delivered, a breathing tube was placed through her mouth. Immediately after birth, she was taken by ambulance to Starship to have a breathing hole made in her neck - a tracheostomy - which would remain until she was aged 5.
Bodee's tracheostomy was made at 17 months, and closed up just before his third birthday.
Kayla was fed through a nose tube into her stomach until she was 8 months old, when her stomach was connected by a tube to a surgical opening on her abdomen, which is how she still feeds herself formula.
These operations and numerous treatments to help Kayla and Bodee cope with their unusually small ear canals were done at Starship.
The periodic Middlemore operations, which will continue until the pair are fully grown, extend the lower jaw. The bone is cut on both sides and an external frame is temporarily screwed on and expanded daily to draw the lower jaw forward. New bone grows into the gap.
Mrs Nield says Kayla can eat some soft foods orally, such as icecream, and it is hoped that after further operations and a procedure to line up her teeth properly, she will be able to eat more solid food.
However, she and husband Chris have always encouraged Kayla to try various foods and she is adventurous and will eat some unusual things that she can soften by sucking them thoroughly.
"She loves raw onion ... and she likes horseradish sauce."
The Nields wrote to the Herald to express their thanks for the Help Our Kids campaign to raise money for Starship's new operating theatre and the refurbishing of four other theatres.
Kayla told the paper, when asked about Starship: "They're really helpful and really nice to everyone at Starship."
Her parents wrote: "We love Starship and encourage everyone to donate a little something to keep the place filled up with up-to-date technology."
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