I think there are a few problems. First, as with statutory rape, I think incest is often treated as de-facto coercive - e.g. could you say "no" to your dad if you depend on him for food/shelter/clothing? Second, rape often requires the victim to accuse the rapist of such - which is probably less likely when family is involved. Finally, incest carries increased risks of abnormalities in the child, which makes the case for aborting to prevent suffering of the child stronger.