Incest refers to sexual intercourse between individuals who are genetically related, such as brother and sister, uncle and niece, or father_and_daughter. This does not include adopted children or people who are only relatives-in-law.
An important part of the topic of incest (at least heterosexual incest) is the concept of incestuous reproduction, also known as in-breeding. Although the offspring of a parent and child has a very high chance of birth defects or deformities, contrary to popular belief, the offspring of siblings are unlikely to show any such birth defects unless those siblings are already the product of several generations of in-breeding.
Further out than that, and the risk of defects becomes almost negligible; cousins who have a child together, for example, have almost exactly the same chance of birth defects as any two unrelated people.