Acquired AA patients will receive the experimental regimen of fludarabine with dose-reduced cyclophosphamide, with results in this prospective single arm experimental group evaluated in the context of our institutional historical experience using HD Cy regimens as well as published outcomes using both fludarabine and high-dose cyclophosphamide-based regimens for MRD-BMT in aplastic anemia. iBMF syndrome patients will receive one of two fludarabine-containing regimens based on disease characteristics, and our outcomes will be compared to previously published data using a variety of regimens. Graft versus host disease (GvHD) prophylaxis will consist of cyclosporine/tacrolimus alone for patients with acquired AA or cyclosporine/tacrolimus plus mycophenolate for patients with iBMF syndromes. For both acquired AA and iBMF syndrome patients, donor chimerism will be assessed at scheduled intervals following BMT and will be used to define patients with full donor or mixed chimerism for comparisons of survival, graft failure, cytogenetic, GvHD, and immune reconstitution outcomes.