A 10-month-old baby boy is to donate his bone marrow to save his big sister's life.
Jacob Davis will have gruelling transplant surgery before his first birthday to help his older sister Seren-Rose, two.
She is among just 150 children nationwide who suffer from a rare disease which damages the body's cells.
Their mum Elouise, 34, said: "The way I look at it is, he will have a couple of days of soreness but he will have a lifetime of knowing he has given his sister the gift of life."
Doctors told the family Seren-Rose, who already has brain damage and learning difficulties, would have died of the condition.
But the bone marrow transplant will prevent any further problems and save her life.
Elouise and the children's dad Simon were tested but were not suitable donors – but Jacob was found to be the perfect match.
Elouise said: "When I got the news that Jacob was a match, part of me was a little sad because I do not want him to undergo any discomfort. We've been told he will be sore for a couple of days.
"I think when they grow up they will have a special bond. I am proud of both of them. I think Jacob will be very protective over her safety and really proud of what he has done for her."
Simon, 48, added: "We are elated - it is better if the marrow comes from a sibling rather than from another donor. The recovery time will be quicker and the chance of it 'taking' is a lot higher.
"She is a lovely little girl, she is always smiling and always laughing. She loves the park and going on the swings."
Seren-Rose was diagnosed with the rare disease mucopolysaccharidosis in December.
Sufferers either have a lack of enzymes - responsible for chemical reactions in cells - or produce enzymes that do not work. This can cause a range of serious health problems and affect mobility and brain development.
The life-changing transplant should prevent Seren-Rose from deteriorating further. The family hope that one day their little girl will learn to talk and might even be able to go to school.
The siblings will have the operations at Great Ormond Street Hospital in central London in May.
Jacob will spend four days in hospital recovering after donating bone marrow while Seren-Rose will need two months of care.
There is an 85 per cent success rate with the transplant, a 5 per cent rejection rate and a 10 per cent chance Seren-Rose could get a life-threatening infection.
Elouise, from West Mersea, Essex, added: "Without the transplant it is a fatal condition where the child regresses. There is no cure but it will mean she won't deteriorate anymore."
Seren-Rose's family have been helped by the MPS Society, which supports people suffering from mucopolysaccharide diseases.
Chief executive Christine Lavery said: "I don't think anybody would want to predict the outcome but we are hopeful it will prove successful.
"Transplants are the only option - the problem we face is Seren-Rose is over the age they would normally do transplants but I welcome that the hospital are doing this for her.
"Transplants certainly prolong life to the point where people can live to a normal lifespan but without a transplant, lifespan would be very short - six or seven years."
Dad Simon will raise funds for the charity by completing a 24-hour radio broadcast.
He said: "It's the only charity which helps families with children with the condition.
"They do so many good things for the children. I thought I would try and give something back."