Eh?? Once is an honest mistake, twice is stupidity, but to repeatedly do it is inviting misery. Seriously, people who are genetically related aren't supposed to have sex again and again because they're at high risk of producing an offspring with multiple mutations.
It's not just low IQ or missing limbs we're talking about here. Inbreeding amongst the royal families in Europe have led to widespread cases of hemophilia in the Royal Princes. Other descendants suffered mental health problems, including schizophrenia (see The Madness of King George).
Although mythology teaches us that Egyptian deities Osiris and Isis were brother and sister before they became husband and wife, these stories aren't really applicable in real life. First of all, they're gods and goddesses. It's assumed they were born as perfect beings. When they mate, they're expected to produce another perfect being with extraordinary abilities.
However, there are also cases of accidental incest between siblings who were separated at birth or between a parent and a child who was put up for adoption immediately after birth. Later, when these long-lost relatives met each other as adults, they felt an intense sexual attraction towards each other. If these people had grown up together starting from childhood before adolescence, then the anti-incest mechanism kicks in and repels any attraction between siblings or blood relatives with feelings of disgust.
Behavioral scientists think this unusual attraction is an egocentric pull between two individuals with the same genetic code. The impulse to get to know the other person more deeply until an intimate bond is formed seems too strong and often misinterpreted as sexual desire. In most cases, both parties eventually succumbed to overpowering lust and consummated this desire. Naturally, those who were aware they're related experienced conflicting emotions afterwards.
If, at one point, these pairs of siblings or parent and child decided to get married, it's of course impossible in most countries, including the united States, where there are laws prohibiting marriage between individuals of first-degree consanguinity (blood relationship). This means marriage is not allowed between uncle-niece, aunt-nephew, brother-sister, parent-child, grandparent-grandchild, and first cousins. In comparison, countries like Japan, India, Pakistan and the Middle East have in their historical records up to 30% of marriages considered as consanguineous.
Once the authorities discovered your relationship, the legal system has the right to imprison you wherever incest is illegal. You'll get separated and denied any chance of seeing each other for a long time. That has happened before somewhere in Germany where a brother (adopted when he was a child) met his sister when he was already 20 yrs old. They fell in love as adults after their mother died and lived together as a couple.
The couple already had four children before they were discovered by authorities and the brother was sentenced to 3 years in prison in 2007. They tried to appeal to the German courts for the lifting of the ban on incest, but they lost in 2008. In 2012, the high courts in Europe upheld the decision of Germany to keep the status quo on incest laws.