Sure, but the excellent and rational driver would know where that limit is and keep within it.
How about, if we're working on an assumption of fully consenting adults, wouldn't that also be an argument against rape laws too?
I get that, but you're asking about changing the law, and the law doesn't only apply to a defined subset of hypothetical situations, it has to apply to any possible situation.
You could make an argument about the moral objections to incest in certain circumstances, and you could even argue for relaxation or mitigation round the existing laws in different jurisdictions, but if you're talking about a simple question of legalising any and all consensual adult incest, you need to consider all of the possibilities.
If birth control was easy and effective, there wouldn't be so many unwanted or unexpected pregnancies. Plus, you assume your hypothetical couples want to use birth control. They could want to have children, either not knowing or not caring about the associated risks.