The question is unclear, in my opinion. The signal bandwidth of the SD ADC doesn't play a role for an anti-alias filter, because it's achieved in the digital domain. It's only important in so far, that the lowest frequency component that has to be supressed by the anti alias filter is fs - 8 kHz. Furthermore, harmonics of fs have to considered as well.
So the problem is about supressing fs components (and it's multiples) from the input signal. Usually SD ADCs have external filters for it, but for a rather low resolution of 12 bit, an integrated analog lowpass filter should be feasible without breaking the noise specification, I hope.