From the Council of Mainz, A.D. 813

Canon LIV. We forbid that anyone hereafter be coupled in marriage in the fourth generation; but where, after the prohibition has been made, it shall have been found, let it be separated.

Cf. Canon I of the Synods of Vermerie and Compiegne. Here that is no longer tolerated which there was permitted.

Canon LV. Let no one receive his own daughter or son from the font of baptism; nor lead as wife a goddaughter or co-mother, nor her whose son or daughter he has led to confirmation. But where this has been done, let it be separated.

Cf. Response IV of Stephen II and the Roman Council under Gregory II.

Canon LVI. If anyone has led a widowed wife, and afterward has fornicated with his stepdaughter, or has married two sisters; or if any woman has married two brothers, or [has been] with a father and son: we command such couplings to be anathematized and disjoined, and never again to be coupled in marriage, but to be placed under great strictness.

By the words "and afterward has fornicated with his stepdaughter," I understand that marriage attempted with her after the death of the wife or of the mother of the same woman is signified. For dissolution is commanded, as if a bond of marriage could not exist between such persons. Moreover, the same was established in the Council of Mainz under Rabanus, A.D. 847; see below; and in the same manner it was answered by Nicholas I to Charles the Archbishop; see below; who indeed was not ignorant that by subsequent incest the bond is not dissolved. You could indeed suspect that perpetual separation alone is commanded; but since here, and by Rabanus, this case is composed together with the case in which someone has married two sisters, etc., and both are subjected to one sanction, while for the others it is certain that there is a diriment impediment, it must be held that also in the former case the discourse is about the nullity of the bond.

You have here also the impeding impediment of incest. Cf. besides Canon XI of the Council of Arles, above.