I think the one main concern with incest laws would be whether or not there was consent. It would have to be between two consenting adults. Yet, I feel like there would be a lot of "he said" "she said" when confronted about it. I also feel like people would abuse the law and attempt it with a child. The law would have to be strict, and there would have to be a way to make sure people aren't abusing it.
Incest between consenting adults should be permitted and excellent expression of love! I don't think the genetic problems are really significant. There is the benefit of keeping the good genetics within the "Family" which in the past some Royal families did. In this country we have freedom. Freedom of expression of love and sex should be free. Sons in love with Moms, Daughters in love
with Dad's. Brother and sister connections are wonderfully satisfying.
As for cousins -- Don't even worry.
Unfortunately the Laws are antiquated.
Adult incest should not be okay under any means. The human body was not built to be cohesive with relatives, it does not work properly that way. If two adults of the same family had a kid they are very likely to end up with a brain disability, or worse. It is unfair to do to future children.