Melphalan and Busulfan Followed By Donor Peripheral Stem Cell Transplant, Tacrolimus, and Methotrexate in Treating Patients With Multiple Myeloma

RATIONALE: Giving chemotherapy, such as melphalan and busulfan, before a donor peripheral blood stem cell transplant helps stop the growth of cancer cells. It also helps stop the patient's immune system from rejecting the donor's stem cells. When the healthy stem cells from a donor are infused into the patient they may help the patient's bone marrow make stem cells, red blood cells, white blood cells, and platelets. Sometimes the transplanted cells from a donor can make an immune response against the body's normal cells. Giving tacrolimus and methotrexate before or after transplant may stop this from happening. PURPOSE: This phase II trial is studying how well giving melphalan together with busulfan followed by donor peripheral stem cell transplant, tacrolimus, and methotrexate works in treating patients with multiple myeloma.