Incest tends to involve someone older imposing upon their own children or younger siblings, cousins, etc. While plenty may involve people of similar age and equal consent, the dynamic is more often than not one involving power & influence a degree of mainpulation or coercian, even if not to an extreme degree. The kissin' cuzins in the barn scenario is an overused trope.
So that as a form of abuse which may or may not border on rape or qualify as such is part of the issue itself.
Genetic abnormalities is another, and given the cultural aspect of such behavior, chances are it's trans-generational and thus the chances for genetic defects to rise gets higher with successive generations, a phenomenom which has been observed in certain areas of the US.