Canon 54 First cousins and certain in-law pairs may not marry

Since the divine Scripture thus clearly teaches us, You shall not go in to any that is near of kin to your flesh, to uncover his shame, the God-bearing Basil enumerated in his own canons some of the forbidden marriages, passing over most in silence, and in both ways securing what is profitable for us. For, declining the multitude of shameful names, so as not to soil his discourse with the words, he comprehended the impurities under general names, through which he pointed out to us comprehensively the unlawful marriages. But since through such silence, and through the indistinctness of the prohibition of lawless marriages, nature confounded itself, we have seen fit to set forth more nakedly the matters concerning this, decreeing from now on that he who is joined in the fellowship of marriage to his own first cousin; or a father and son to a mother and daughter; or a father and son to two maiden sisters; or a mother and daughter to two brothers; or two brothers to two sisters, fall under the canon of seven years, on their openly separating from the unlawful cohabitation.