Incest is a repugnant subject for most people because of its forbidden nature. Research shows that this reaction is more cultural than it is biological. There are a couple of mechanisms found that explains how it happens. The first is the Westermarck effect, named after Edvard Westermarck, who wrote about it in 1891.
The idea of the Westermarck effect is that young children will become sexually/romantically desensitised to anyone they live in close contact with over the course of the first few years of their lives. That is, they will reach adulthood with no compulsion to consider a relationship with anyone they shared a home with in their early childhood. Note that crucially, the connection does not have to be biological; according to the theory, it applies just as readily to children adopted at a young age as to those raised by their birth parents.
It also explains why unrelated children raised together in communes, or even students who spent years together in small schools, tend to date each other less than they date those outside those communities. That idea of the Westermarck effect gains even more credence when we consider the opposite, which is called genetic sexual attraction, or GSA.
Genetically related individuals who are not raised together often fail to be sexually and romantically blind to each other. That is, when a pair of biologically related individuals meet for the first time in adulthood, they often find each other very attractive. Genes ensure that the two have a lot in common, and the absence of the Westermarck effect sometimes makes them difficult for one another to resist.
There are quite a few examples of GSA in modern times, when long-lost relatives have the means to find each other after many years apart. Read more about both these effects at Damn Interesting. Link