RATIONALE: Giving low doses of chemotherapy before a donor stem cell transplant helps stop the growth of cancer cells. It may also stop the patient's immune system from rejecting the donor's stem cells. The donated stem cells may replace the patient's immune cells 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 antithymocyte globulin before the transplant and tacrolimus and methotrexate after the transplant may stop this from happening.