RATIONALE: Giving low doses of chemotherapy, such as fludarabine, and radiation therapy before a donor bone marrow or 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 cyclophosphamide, mycophenolate mofetil, and tacrolimus after transplant may stop this from happening.