Beyond the potential biological issues, the biggest problem tends to be coercion and consent. The majority of incestuous encounters are abusive and involve a power dynamic that makes informed consent impossible.
Now, if every party is an adult and capable of informed consent, it is possible to test for likelihood of genetic defects based upon the parents’ genes. So, I can’t think of a non-subjective objection if, for example, they met for the first time as adults and didn’t know of such relation. Still pretty weird to me but I don’t think it’s anyone’s place to interfere with healthy, loving relationships.