This phase II trial studies the outcomes of using a JAK inhibitor prior to reduced intensity haploidentical (Haplo) transplantation for the treatment of primary or secondary myelofibrosis (MF). Haplo transplant has been shown to be safe and effective in patients with leukemia and lymphoma who don't have an available sibling donor. The primary risk of using Haplo HCT in patients with MF is graft failure as the graft failure rate has been historically higher with Haplo HCT than with other donor sources and higher with MF patients due to bone marrow fibrosis than in patients with other hematologic malignancies. JAK inhibitors when used in patients with MF may decrease the size of the spleen and decrease inflammation in the bone marrow. Therefore using a JAK inhibitor prior to Haplo transplant has the potential to decrease graft failure in patients with MF. Haplo transplants for patients with MF have been done successfully at multiple institutions in patients not on a study and are currently being covered by Medicare.