The real reason why familial incest is taboo is because it makes it more difficult to make money.

The real reason why familial incest is not legal is because it leads to marriages with less social status and less inheritance, which makes it more difficult to make money. The real reason why incest is not legal is because it hurts the economy. The wealthy elite, who control the government, want to prevent sibling marriage because it prevents the wealthy, powerful, and intelligent from marrying their children off to other wealthy and intelligent families to increase their wealth, power, and intelligence of their offspring. It makes it difficult to create an elite class when people are copulating with their direct relatives instead of marrying outside. If you are just having incestuous relations with your siblings and not marrying them, then it doesn't affect the economy. It is also damaging to the economy to have people spending their money on condoms and birth control instead of spending their money on alcohol, drugs, tobacco, fast food, restaurants, and strip clubs. The government doesn't want people having incestuous relations which could lead to pregnancy and could lead to the person having less money to spend on vices. The government also has a special interest in keeping people separated so that they are lonely and more likely to spend money on pornography, video games, movies, and other things to take their mind off of their loneliness. It is easier to control a less intelligent, less powerful, and less wealthy population. If incest were legal, people would be having more sex to cope with their loneliness, but this sex would not be generating any profit. Legal incest would lead to a decrease in overall wealth and power and the government cannot allow this to happen.