This is a phase II study of atorvastatin for the prophylaxis of acute GVHD in patients undergoing matched-sibling allogeneic HSCT. This study will explore a two-pronged acute GVHD prophylaxis strategy, consisting of pre-treating consenting sibling donors with atorvastatin before stem cell collection, followed by the addition of atorvastatin to methotrexate/tacrolimus-based GVHD prophylaxis.