This is a pilot study to determine the safety and feasibility of the COH-MC-17 regimen and ability of the regimen to induce a mixed chimeric status in severe sickle cell disease patients (hemoglobin SS or S-βº Thalassemia). The COH-MC-17 regimen consists of a non-myeloablative regimen (cyclophosphamide, pentostatin and rabbit-anti-thymocyte globulin (ATG)) followed by a CD4+ T-cell-depleted haploidentical hematopoietic cell transplant (HaploHCT).