The related donor stem cell transplantation using the alemtuzumab/TBI platform has been shown to be a safe strategy to cure severe sickle cell disease. However, due to a lack of suitable donors, many patients cannot benefit from this strategy. Alternative donor sources are desperately needed to fill this gap. Nearly all patients will have a haploidentical family member who would be able to donate. The use of post transplantation cyclophosphamide has greatly improved the outcome of haploidentical stem cell transplantation. The investigators propose to combine this with alemtuzumab/TBI conditioning.