This is fairly simple to figure out.
There has been medical science behind proof of genetic problem or defects, in children born through incest conception, direct relative, as in son and mother, daughter and father and sister and brother. This information has been made available for over 30+ years.
(1) In an animalistic society, incest is proper and to be expected. This uses basic sexual interest to control life, instead of conscience. You would tend to mate like animals.
(2) In semi-civilized society, incest is defined as immediate family, up to and including first cousins. After that, there is no restriction, as per law. This is American society. Incest is illegal, as per, Federal authorities. This varies from Country to Country.
Hope I cleared that up.