Father Thomas Berg LC neatly elucidates why the creation of so-called "savior siblings" is wrong::
First of all, it would commonly entail recourse to IVF though couples are known to have conceived a savior child naturally prior to the advent of [preimplantation genetic diagnosis] for screening embryos. As I’ve noted before, IVF is immoral because through it, a child is manufactured as the result of a technician’s hand, rather than being generated through the unitive love of the child’s parents.Read the whole essay.
A second element of the moral disorder here is that this process requires the manufacture of multiple embryos. All the embryos are screened using PGD – a process which exposes the developing embryos to some risk – by removing one or two cells from each embryo in order to conduct the genetic testing necessary to determine whether the embryo is a match. When a genetic match is found, there remain multiple developing embryos – all of them genetic siblings to the first child – who are either frozen, destroyed in research, or simply “discarded.” ...
A third element of the moral disorder is the way it reduces newly conceived human life to the level of an object. The mere fact that a child’s very existence is solely due to his or her ability to cure an older sibling is stunningly utilitarian. Parents of savior children reassure critics that they love the specially-created child as much as any. Nonetheless, the child is brought to term on the calculated determination of his or her usefulness in curing an older sibling. But for that, the child would not exist.