Incest
Incest is when two people who super closely related by blood, love each other very much. The concept of incest is fundamentally disgusting and is of course condemned by various religions like the Abrahamic religions and Buddhism (Unfortunately that didn't stop hillbillies and rednecks from fucking their siblings and cousins).
Unfortunately many stinky atheists are starting to become sexually attracted to the concept of incest because they developed a crippling porn addiction.
A prime example of an incestuous group, are talking, wildly coloured ponies. Namely Applejack and her retarded family. Someone call Lee Goldson.
Apparently, Isabelle X Digby fans don't think of the consequences of incest. As much as fans of vore don't think of the ethical horror and logistical consequences of devouring a whole person alive.
Consequences
Incest can lead to deformities caused by autosomal recessive genetic disorder which results in generally undesirable offspring, making them more prone to disease and deformities such as having two heads, having one eye, having a bent spine, and mental retardation.