Incest is a crime.
Even between consenting adults (and shortly after the Supreme Court decision on sodomy such a case was prosecuted in California between non-related brother and sister, I think at least one was adopted).
The left has used incest as one of the reasons that parents should not be notified of a daughter seeking an abortion. That defense makes no sense as it shields an abuser from prosecution and destroys the evidence.
BTW, there are STILL back alley abortions (they never were carried out in alleys) and women still are injured or even dying from having the procedure done in licensed offices.
I seriously doubt that laws banning incest between consenting adults would hold up to a court challenge. It certainly doesn't make much sense in light of the sodomy rulings.
Personally, I couldn't give a crap less if a brother/sister father/daughter son/mother or whatever wants to sleep with each other as long as they don't ask for any special favors for it.