A baby boy is set to save his sister's life by giving her his bone marrow.
Seren-Rose Davis, 2, was diagnosed with a metabolic disorder in December. She is unable to speak and her condition has left her with brain damage and learning difficulties.
Her parents, Elouise and Simon, were told that if Seren-Rose was left untreated, she would be unlikely to live to adulthood.
Neither of them were suitable donors, but son Jacob, who will turn one in June, did have the bone marrow match.
Maintenance manager Mr Davis said: "She's a lovely kid who's always smiling and is happy for her little brother to push her about. When she was diagnosed in December we were told that the disease usually ends in imminent death."
"Fortunately she has the least severe type and it is treatable - but her chances of survival without a transplant would be slim," Mr Davis said.
Adults and older children would normally undergo an assessment to make sure they were certain of their decision and were not being pressured into it.
This has not been possible because of Jacob's age but his parents said they were sure they were making the right decision.
"Obviously he isn't old enough to give consent and some people would criticise us for doing this but the pros outweigh the cons," Mr Davis added.
The couple, from West Mersea, Essex, are now awaiting surgery on both their children.
The surgery will take place at the end of this month and will be followed by Seren-Rose undergoing a week of intense chemotherapy before spending six weeks in isolation.
Jacob is likely to be in hospital for about four days and will experience some short-lived discomfort.
If the surgery is successful Seren-Rose is expected to live to 40 or 45 years old, although her development will always be impaired.