Incest is a major taboo in society and we all know what it is. Since it is a social taboo is very much all cultures, it is therefore an example of 'being bad'. It involves any sexual activity between close relatives such as sisters and brothers, however, some societies consider incest differently. Some see it to include only those who live in the same household, whilst others consider it to include people related by blood and other societies include those related by adoption or marriage. When incest involves an adult and a child (e.g father and daughter) it is also classified as a form of child sexual abuse and is illegal in every developed country.
Although the actual incestual interaction is very rare, the thought of it may be more common then we may be aware of. This is suggested by the psychologist who we all know...Sigmund Freud! He proposed the psychoanalytical theory, The Oedipus Complex which is when a boy is sexually fixated on his mother and therefore, competes with his father for his mother's love. This can also occur in girls as well, and is called The Electra Complex, which is when the girl is sexually fixated on her father.
"His destiny moves us only because it might have been ours – because the oracle laid the same curse upon us before our birth as upon him. It is the fate of all of us, perhaps, to direct our first sexual impulse towards our mother and our first hatred and our first murderous wish against our father. Our dreams convince us that this is so." - Freud speaking of the Oedipus myth.
So yes, incest is definately bad! But what about consented incest, where both of the individuals actually want to have a incestual relationship? Should society be punishing people in these situations? There are no real answers to this because people tend not to acknowledge it and let the law run its course.
More information can on incest can be found on the links provided below (I wouldn't usually use Wikipedia to research on a topic, however it is very difficult to find any relative links that provides factual information when the internet is flooded with 'videos', in which I did not click on!):
The link above is actually a article on a woman's postive account of incest with her brother.