Personally, I do not think incest should be illegal. I think simply having a child from incest should be illegal. It should be the law that if you are going to be in an incestuous relationship then you must agree to use some form of guaranteed birth control.
A condom is not enough. Either implanted control or vasectamony. If a child is still born after this no charges, as all reasonable steps were taken, however if it turns out they were lying about using birth control, or simply used none at all, they should be criminally charged and the child should be taken into surrogate care on birth.
Objectively the only thing wrong with incest is that the children are far more likely to be born with serious defects. It is not fair on the child, rather than the parents if the child is born. Therefore we erase that and even though it is weird and creepy (in my view), a lot of legal things are.