Taboos are created by society.
In Pharoas of Egypt, most common marriages were between brothers and sisters.
Marriages between first cousins are taboo in USA.
People tell me they were common in Europe.
These are the most common arranged marriages in oriental countries.
Among animals, there are no such taboos.
You don't see increased risk of congenital defects in animals.
There was an article about mathematical model in journal of genetics. If marriages between siblings were legally allowed, there was no increased risk of genetic defects.
You eat pork every day.
Just the smell of pork makes me throw up.
That is social learning.
I have no sister, so I have no way to judge this issue.
I am a hypocrite. I don't believe in casual sex. But I do engage in it.
My belief is that if two people are in love, they should be allowed to get married. Even if they are brothers and sisters.
You are allowed to throw up on this statement.