This phase II trial studies how well busulfan, fludarabine, donor stem cell transplant, and cyclophosphamide in treating patients with multiple myeloma or myelofibrosis. Drugs used in chemotherapy, such as busulfan, fludarabine, and cyclophosphamide, work in different ways to stop the growth of cancer cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Giving chemotherapy before a donor stem cell transplant helps stop the growth of cells in the bone marrow, including normal blood-forming cells (stem cells) and cancer 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. Giving busulfan and fludarabine before and cyclophosphamide after donor stem cell may work better in treating patients with multiple myeloma or myelofibrosis.