The purpose of this study is to investigate host myeloimmunosuppressive conditioning followed by familial haploidentical T cell depleted allogeneic stem cell transplantation in patients with high risk Sickle Cell Disease (SCD). It is hypothesized that it will be safe and well tolerated, and result in sustained donor chimerism, acceptable engraftment and immune reconstitution. Also, that it will limit SCD related organ damage resulting in improved and/or stable neurological, neurocognitive, pulmonary and pulmonary vascular function and health related quality of life (QOL).