Mrs Nash underwent four IVF cycles before becoming pregnant with Adam and each time embryos were screened to eliminate those carrying the Fanconi anaemia gene.
Already questions are being raised about whether the baby boy was really wanted or merely "created as a medical commodity" to save his sister.
Doctors genetically tested the embryos of an American couple before implanting one of them in the mother's womb.
They chose the embryo that would have the exact type of cells needed to save the couple's six-year-old daughter, who is suffering from a life-threatening bone marrow deficiency.
The use of the screening technique is likely to rekindle the debate over genetic testing and medical ethics, with "pro-lifers" claiming the science involved is becoming "more and more monstrous".
Lisa and Jack Nash's first child, Molly, was born with Fanconi anaemia, a genetic disorder leading to failure of bone marrow production.
When trying for a second child by IVF, doctors used preimplantation genetic diagnosis (PGD) in order to select a healthy embryo which did not carry the disease.
Selected for transplant match
But, in this case, doctors also selected an embryo which would produce the tissue match necessary to offer Molly the chance of a bone marrow transplant.
PGD allows scientists to study embryos following IVF and select and implant those which do not carry specific faulty genes.
Mrs Nash underwent four IVF cycles before becoming pregnant with Adam and each time embryos were screened to eliminate those carrying the Fanconi anaemia gene.
The resulting boy, Adam, born in August, has now been able to donate stem cells collected from his umbilical cord to his older sister to replace her failing bone marrow.
The procedure took place late last month at Fairview-University Medical Center in Minneapolis and doctors there believe Molly now has an 85-90% chance of recovery.
Children with Fanconi anaemia suffer from severe bleeding and immune system disorders and invariably die by the time they reach eight or nine.
The only effective treatment is a bone marrow transplant from a perfectly matched healthy sibling.
Not allowed under UK law
The Head of ethics and policy for the British Medical Association, Dr Vivienne Nathanson, suggested the technique would not be allowed under British law because of the possibility the child was being seen simply as a "medical product".
In the UK, PGD has been used in five clinics for the past 10 years. The technique has resulted in the birth of about 20 healthy babies, who would otherwise have been at risk of serious genetic diseases such as cystic fibrosis or haemophilia.
James Yeandel, spokesperson for the regulatory body the Human Fertilisation and Embryology Authority, said that an application for use of PGD for a purpose such as creating a transplant match would have to be considered by the authority's licensing committee.
But he added: "Use of PGD has been approved for a number of serious genetic diseases on a named disorder basis. It can not be used for any social, physical or psychological reasons."
And the selection technique has been vigorously condemned by pro-life organisation Life.
Life spokesman Kevin Male added: "Adam was the fifteenth embryo created which meant fourteen people were killed before him. In essence a white coated technician brought this human being into the world simply as a means to an end."