Objection 1. It would seem that affinity is not an impediment to marriage. For nothing is an impediment to marriage except what is contrary thereto. But affinity is not contrary to marriage since it is caused by it. Therefore it is not an impediment to marriage.
Objection 2. Further, by marriage the wife becomes a possession of the husband. Now the husband's kindred inherit his possessions after his death. Therefore they can succeed to his wife, although she is affine to them, as shown above (Article 5). Therefore affinity is not an impediment to marriage.
On the contrary, It is written (Leviticus 18:8): "Thou shalt not uncover the nakedness of thy father's wife." Now she is only affine. Therefore affinity is an impediment to marriage.
I answer that, Affinity that precedes marriage hinders marriage being contracted and voids the contract, for the same reason as consanguinity. For just as there is a certain need for blood-relations to live together, so is there for those who are connected by affinity: and just as there is a tie of friendship between blood-relations, so is there between those who are affine to one another. If, however, affinity supervene to matrimony, it cannot void the marriage, as stated above (Question 50, Article 7).
Reply to Objection 1. Affinity is not contrary to the marriage which causes it, but to a marriage being contracted with an affine, in so far as the latter would hinder the extension of friendship and the curbing of concupiscence, which are sought in marriage.
Reply to Objection 2. The husband's possessions do not become one with him as the wife is made one flesh with him. Wherefore just as consanguinity is an impediment to marriage or union with the husband according to the flesh, so is one forbidden to marry the husband's wife.
Source: Summa Theologica (New Advent)