The divine scriptures plainly teach us as follows, “Thou shalt not approach to any that is near of kin to thee to uncover their nakedness.” Basil, the bearer-of-God, has enumerated in his canons some marriages which are prohibited and has passed over the greater part in silence, and in both these ways has done us good service. For by avoiding a number of disgraceful names (lest by such words he should pollute his discourse) he included impurities under general terms, by which course he shewed to us in a general way the marriages which are forbidden. But since by such silence, and because of the difficulty of understanding what marriages are prohibited, the matter has become confused; it seemed good to us to set it forth a little more clearly, decreeing that from this time forth he who shall marry with the daughter of his father; or a father or son with a mother and daughter; or a father and son with two girls who are sisters; or a mother and daughter with two brothers; or two brothers with two sisters, fall under the canon of seven years, provided they openly separate from this unlawful union.
Ancient Epitome of Canon LIV.
Thou shalt not permit the marriage of a son of a brother to the daughter of a brother; nor with a daughter and her mother shall there be the marriage of a son and his father; neither a mother and a daughter with two brothers; nor brothers with two sisters. But should anything of this sort have been done, together with separation, penance shall be done for seven years.
Reproduced in full. Translation: Nicene and Post-Nicene Fathers, series 2, vol. 14 (Percival, 1900), public domain (Public domain).