I never really looked into it, but those who oppose incest have never shown me any studies. I've heard of an argument about mixing the same DNA is what can cause issues, but the argument seemed to leave out the fact that no DNA is actually alike (except in identical twins, which are exact copies - I know of no studies made on incestuous twins, and they do exist: twin lesbians (making out with each other), for example, is a thing in porn tubes).
I'm also suspicious of all of that because, if you google "father daughter had baby" or "incest baby" or whatever, you'll find quite a few cases of incestuous births, but you'll also find that what people are talking about doesn't seem to have any mentions of birth defects or anything of the sort. Instead what you find is plenty of moral and legal concerns. News headlines are always only bothered about whether it's morally right or wrong, whether they should be arrested or not, and whether they should keep the baby or not, etc.