Marriages or any form of sexual relations between blood relatives is incest and is completely prohibited in the United States and most other countries of the world.
There are two reasons for this:
1. Religious: Most major world religions prohibit marriages between blood relatives. In countries where church and other religious institutions had a strong following, incest remains a crime.
2. Biological: Medical Science has proven that off-springs of close relatives e.g. brother-sister, first cousins and even second cousins have a high chance of genetic mutation which can lead to various health issues. This is the main reason why modern societies prohibit incest