Brief analysis: (not necessarily my opinion, just throwing ideas out there)
The government can only legislate effectively against incestuous relationships, it cannot effectively legislate against unprotected sex between people in incestuous relationships, only the results of this unprotected sex (i.e. the embryo, fetus, child, etc.). Legalising incestuous relationships will make it more likely for a given person to be in incestuous relationship. This in turn raises the rate of embryos being concieved through incestuous relationships to increase. Fact is, unprotected sex does occur. Also, contraceptives are not infallible. They can fail. This means that legalising incestuous relationships will increase the amount of embryos concieved by the two people in the relationship. The embryos concieved will either be kept, or aborted. See http://en.wikipedia.org/wiki/Inbreeding for the effects on inbreeding on an individual or population. Abortion is a complex issue. Many people will oppose the abortion of these embryos. Abortion therefore should ideally be avoided. Keeping incestuous relationships illegal will minimise the need for these abortions, as demonstrated earlier.
This issue is too complex hazy to properly draw conclusions.