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.

Fair point, I meant more for the nonconsensual aspect, although I'm unsure if incest is illegal even if consensual, I guess it might depend on the jurisdiction, at which point eve if it were two people consenting, it may not legally be considered consensual, similar to how people under a certain age have no legal ability to consent.