I’m pretty sure there’s a canon law that says, if you marry A, you can’t go on to marry A’s brother if he dies. That’s one of the things that came up when Henry VII wanted his son, the future Henry VIII, to marry Catherine of Aragon, who had already been married to Henry VII’s older son, Arthur, for about five or six months before he died. They had to get a Papal dispensation for it. Catherine had to swear that the marriage had never been consummated, etc., and therefore was not an impediment to her marriage with Henry VIII. But even so----
Can. 108 §1. Consanguinity is computed through lines and degrees.
§2. In the direct line there are as many degrees as there are generations or persons, not counting the common ancestor.
§3. In the collateral line there are as many degrees as there are persons in both the lines together, not counting the common ancestor.
Can. 109 §1. Affinity arises from a valid marriage, even if not consummated, and exists between a man and the blood relatives of the woman and between the woman and the blood relatives of the man.
§2. It is so computed that those who are blood relatives of the man are related in the same line and degree by affinity to the woman, and vice versa.
Can. 1091 §1. In the direct line of consanguinity marriage is invalid between all ancestors and descendants, both legitimate and natural.
§2. In the collateral line marriage is invalid up to and including the fourth degree.
§3. The impediment of consanguinity is not multiplied.
§4. A marriage is never permitted if doubt exists whether the partners are related by consanguinity in any degree of the direct line or in the second degree of the collateral line.
Can. 1092 Affinity in the direct line in any degree invalidates a marriage.
Can. 1093 The impediment of public propriety arises from an invalid marriage after the establishment of common life or from notorious or public concubinage. It nullifies marriage in the first degree of the direct line between the man and the blood relatives of the woman, and vice versa.
Can. 1094 Those who are related in the direct line or in the second degree of the collateral line by a legal relationship arising from adoption cannot contract marriage together validly.