In the central nervous system (CNS) of Drosophila embryos lacking regulator of cyclin A (rca1) or cyclin A, we observe that several ganglion mother cells (GMCs) fail to divide. Whereas GMCs normally produce two sibling neurons that acquire different fates ('A/B'), non-dividing GMCs differentiate exclusively in the manner of one of their progeny ('B'). In zygotic numb mutants, sibling neuron fate alterations ('A/B' to 'A/A') occur infrequently or do not occur in some sibling pairs; we have determined that depletion of both maternal and zygotic numb causes sibling neurons to acquire equalized fates ('A/A') with near-complete expressivity. In rca1, numb mutant embryos, we observe binary cell fate changes ('B' to 'A') in several GMCs as well. Finally, we have demonstrated that expression of Delta in the mesoderm is sufficient to attain both sibling fates. Our results indicate that the intrinsic determinant Numb is absolutely required to attain differential sibling neuron fates. While the extrinsic factors Notch and Delta are also required to attain both fates, our results indicate that Delta signal can be received from outside the sibling pair.