The investigators plan to offer stem cell transplantation to sickle cell patients with aggressive disease who only have a partially matched HLA sibling donor. Haploidentical transplants are considered only for patients with no other standard options available who would normally be treated with supportive (palliative) care or given the option to participate in a clinical trial. Donors who are HLA-haploidentical will be the source of hematopoietic stem cells. Potential donors can include any relative (e.g. parents, offspring, siblings, cousins, aunts/uncles, grandparents).