Two or more ions are said to be isoelectronic if they have the same electronic structure and the same number of valence electrons.
Na+ = 10 electrons = 2, 8
Mg2+ = 10 electrons = 2,8
O2− = 10 electrons = 2,8
Si2+ = 12 electrons = 2,8,2
implies Si2+ is not isoelectronic with the rest.