This study looks at a special kind of stem cell transplant for kids with sickle cell disease (SCD), a serious blood disorder. The transplant uses cells from a brother or sister who is a blood type mismatch (called major ABO-incompatible). They use a "nonmyeloablative" method, which means the treatment is gentler on the body. The treatment involves medicines like alemtuzumab, total body irradiation (TBI), and sirolimus to help the body accept the new cells. The goal is to make this transplant option safer and available for more kids with SCD.

SCD can cause severe health problems and shorten life. While a stem cell transplant (HSCT) can cure SCD, it's usually only an option for a few patients with a matching sibling donor. This study checks if the new method works and is safe when the blood types don’t match perfectly. They also watch for a problem called pure red cell aplasia (PRCA), which can happen with these transplants.

• The study is for kids aged 12 months to 18 years with specific types of sickle cell disease.
• Participants need a matched sibling donor with a major ABO-incompatible blood type.
• Kids should have ongoing sickle cell problems, even with treatment, to join.