[Iustiniani Institutiones]

Two [people] married [at the same time] cannot be

Likewise, if your stepdaughter is still [your stepdaughter]—that is, if her mother is married to you—you cannot take her as a wife, because it is not permitted to have two wives at the same time. It is also forbidden to take a mother-in-law or a stepmother as a wife, because they are in the place of a mother. This proceeds only when the affinity original: "affinitas" has been dissolved. Otherwise, if she is still your stepmother—that is, if she is still married to your father—you are impeded from marrying her by common law, because the same woman cannot be married to two men.

Likewise, if she is still your mother-in-law—that is, if her daughter is still married to you—marriage to you is impeded, because you cannot have two wives. Nevertheless, a husband's daughter from another wife, or a wife's daughter from another husband, or the opposite, are not prohibited from marriage.

And they may contract [marriage], even if they have a brother or sister born from a marriage contracted afterward. And if your wife, after a divorce, has borne a daughter by another man, this one is indeed not your stepdaughter, but Julian a prominent Roman jurist says that one ought to abstain from such marriages. For it is agreed that neither is a son's betrothed a daughter-in-law, nor is a father's betrothed a stepmother. Nevertheless, they would act more rightly in the eyes of the law who have abstained from such marriages. It is certain that these cognations original: "cognationes" also serve as an impediment to marriage if, by chance, a father and daughter, or a brother and sister, have been manumitted original: "manumissi".

There are also other persons who, on account of diverse reasons, are prohibited from contracting marriage, which in the books of the Digest a collection of Roman legal writings or Pandects an encyclopedic compilation of Roman civil law from the old law...