IT IS the ultimate gift that any sister can give – the gift of life.

Six-year-old Kacey Smith is going to become a bone marrow donor for her younger sister Milly in a bid to cure her of a rare immune disorder that makes even a play in the park or a swim in the sea potentially fatal.

Without it, four-year-old Milly, who suffers from chronic granulomatous disorder, would be unlikely to live past her teenage years.

Their parents, John and Theresa Smith, from New Milton, were devastated when Milly was diagnosed last October and discovered that any infection could be lifethreatening to the little girl.

However, just weeks later, tests to ensure her sister Kacey and one-year-old brother Brody, did not have the disease revealed that Kacey was the perfect match to save Milly’s life.

Theresa said: “We have told Kacey that she has the magic to make her sister better and save her life. It is a lot to take in for anyone, let alone a little girl, but although she is a bit wary of going into hospital she is very proud to be able to help her sister.

“We are blessed that Kacey can do this for Milly. Other families are forced to wait on the long waiting lists to find a match and my heart goes out to them.”

Milly was born with the disorder that means her body cannot fight against fungal bacteria, making her vulnerable to common infections such as a cold and leaving a simple cut open to blood poisoning.

It has meant that Milly cannot play near cut grass or dead leaves – particularly difficult when living in the New Forest – because if she breathes in the spores they could be potentially fatal.

A bone marrow transplant is the only cure for the disorder and although it will mean Milly enduring radiotherapy and chemotherapy to eradicate her immune system before she can receive Kacey’s bone marrow, followed by up to eight weeks in isolation, it will give her the chance to lead a normal life.

Theresa, who was forced to give up work as shop assistant to care for Milly, added: “It will be a tough year for Milly. I have my concerns because it will mean both my girls going into hospital but I know for Kacey the procedure is pretty simple and without it, Milly probably wouldn’t reach her 20s.”

Now Theresa and John, a painter and decorator, are on a fundraising mission to be able to take Milly on her dream holiday to Euro Disney – giving her something to look forward to once she has recovered from her bone marrow transplant.