RATIONALE: Giving low doses of chemotherapy, such as fludarabine and melphalan, before a donor bone marrow or peripheral blood stem cell transplant helps stop the growth of cancer cells. It also stops the patient's immune system from rejecting the donor's stem cells. The donated stem cells may replace the patient's immune system and help destroy any remaining cancer cells (graft-versus-tumor effect). Sometimes the transplanted cells from a donor can also make an immune response against the body's normal cells. Giving tacrolimus and methotrexate after transplant may stop this from happening.