Are related donors better for transplants?

The overall answer is yes, but this is a complex topic.

The two important measures of patient outcome are: long-term survival, and the  impact on quality of life from graft-versus-host disease (GvHD). Sibling donors tend to trigger less GvHD.  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.

The exact comparison of outcome between sibling or unrelated donor varies with the patient diagnosis.  The NMDP website has a page on this, with numerous references.  For many cancers the outcomes are comparable, although sibling donors have a slight edge. The largest 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%. Sibling donors show a significant improvement for pediatric cord blood transplants of hereditary disorders.  The European Blood and Marrow Transplantation Group (EBMT) reported 3 year survival rates of 95% from a sibling donor vs. 61% from an unrelated donor.

References:
Weisdorf, D.J. et al. Blood 2002; 99:1971-1977.
Bizzetto, R. et al. (EBMT) Haematologica 2011; 96(01):134-141