Objection 1. It would seem that marriage is not annulled by the husband committing incest with his wife's sister. For the wife should not be punished for her husband's sin. Yet she would be punished if the marriage were annulled. Therefore, etc.
Objection 2. Further, it is a greater sin to know one's own relative, than to know the relative of one's wife. But the former sin is not an impediment to marriage. Therefore neither is the second.
Objection 3. Further, if this is inflicted as a punishment of the sin, it would seem, if the incestuous husband marry even after his wife's death, that they ought to be separated: which is not true.
Objection 4. Further, this impediment is not mentioned among those enumerated above (Article 50). Therefore it does not void the marriage contract.
On the contrary, By knowing his wife's sister he contracts affinity, with his wife. But affinity voids the marriage contract. Therefore the aforesaid incest does also.
Further, by whatsoever a man sinneth, by the same also is he punished. Now such a man sins against marriage. Therefore he ought to be punished by being deprived of marriage.
I answer that, If a man has connection with the sister or other relative of his wife before contracting marriage, even after his betrothal, the marriage should be broken off on account of the resultant affinity. If, however, the connection take place after the marriage has been contracted and consummated, the marriage must not be altogether dissolved: but the husband loses his right to marital intercourse, nor can he demand it without sin. And yet he must grant it if asked, because the wife should not be punished for her husband's sin. But after the death of his wife he ought to remain without any hope of marriage, unless he receive a dispensation on account of his frailty, through fear of unlawful intercourse. If, however, he marry without a dispensation, he sins by contravening the law of the Church, but his marriage is not for this reason to be annulled.
This suffices for the Replies to the Objections, for incest is accounted an impediment to marriage not so much for its being a sin as on account of the affinity which it causes. For this reason it is not mentioned with the other impediments, but is included in the impediment of affinity.
Source: Summa Theologica (New Advent)