While I agree, putting that aside for a moment and assuming for the sake of argument that some allowances dependant upon the circumstances of conception are permissable, I still can see no justification whatsoever for an allowance of abortion in the case of incest.
If the incestuous coupling were non-consensual (likely to be most cases but I'm guessing), then the rape allowance already permits abortion and thus a specific incest allowance is redundant.
If the incestuous coupling were consensual, then what reason is there to terminate the pregnancy? The fear of the possibility (which, contrary to common belief, is most definitely not a certainty) of retardation? But then either an allowance for the abortion of retarded children exists independantly, in which case this is a redundancy, or it does not, in which case such fear is not sufficient reason to terminate the pregnancy.
Simply put, in no way can an incest allowance for abortion be justified.