Are related donors better for transplants?

In general sibling donors are better than unrelated donors for stem cell transplants. The exact comparison depends on the patient's diagnosis and the stage of disease.

The two important measures of patient outcome after a stem cell transplant are: long-term survival, and the amount of graft-versus-host disease (GvHD) that the patient suffers. Sibling donors trigger less GvHD, so that quality of life is better post-transplant. Also, sibling donors are available faster than searching for an unrelated donor, and patients have better survival when they go to transplant faster after diagnosis.

Some case by case studies: For many adult cancers the outcomes of transplants from siblings versus unrelated donors are comparable, although sibling donors have a slight edge. One large study was by Weisdorf et al. 2002, for over 2900 patients with CML leukemia. When correcting for all other factors, the survival with sibling donor vs unrelated donor was 68% vs. 61%. However, in pediatric transplants for hereditary disorders, sibling donors have a distinct advantage. The European Blood and Marrow Transplantation Group (EBMT) reported in 2011 that three year survival rates were 95% from a sibling donor vs. 61% from an unrelated donor.

The donor registry Be The Match has a section of their clinical website which reviews this topic here.

References:
Weisdorf, D.J. et al. Blood 2002; 99:1971-1977. doi:10.1182/blood.V99.6.1971
Bizzetto, R. et al. (EBMT) Haematologica 2011; 96(01):134-141 doi:10.3324/haematol.2010.027839