Ante-Nicene Fathers, Vol VIII:
The Decretals.: Part IV

IV.

(Of marriages among blood-relations, and of those who are born of them; and of accusations which the laws reject.)

Moreover, marriages among blood-relations are forbidden, since all laws, both sacred and secular, forbid such.  Wherefore the divine laws not only expel, but even anathematize, those who do so, and those who spring from them.  Secular laws, again, call such persons infamous, and interdict them from inheriting.  And we too, following our fathers, and keeping close by their footsteps, brand such with infamy, and hold them to be infamous, because they are sprinkled with the stains of infamy.  Neither ought we to admit those men or their accusations, that secular laws reject.  (For who doubts that human laws, when they are not inconsistent with reason and honour, are to be embraced, especially when they either further the public good or defend the authority of the ecclesiastical office, and uphold it as a help?)  And we call those blood-relations whom divine laws, and those of the emperors, both Roman and Greek, name blood-relations, and whom they admit to the right of inheriting, and cannot exclude from that.  Marriages, then, between such are neither lawful nor capable of holding good, but are to be rejected.  (And if any such are attempted in rash daring, they come to be rescinded by apostolic authority.)