From the Supplement, compiled after Aquinas’s death from his Commentary on the Sentences and transcribed here from the 1917 to 1922 printings. About the Supplement.
Text: Summa Theologiae, Supplement to the Third Part (Supplementum Tertiae Partis), transcribed from Internet Archive scans, translated by the Fathers of the English Dominican Province. The Supplement, compiled after Aquinas’s death from his earlier Commentary on the Sentences, in the same English Dominican translation as the Gutenberg Parts. Transcribed by Bibliotheca Catholica from the OCR of four Internet Archive scans of the pre-1931 printings (Questions 1 to 33 from the 1917 Washbourne and Benziger volume, 34 to 68 from the 1922 volume, 69 to 86 from the 1921 volume, and 87 to 99 from the 1922 Benziger volume), with OCR errors corrected and doubtful passages marked, not rewritten. 99 questions and 446 articles. The two appendices printed after Question 99 are not included. Sources, SHA-256 of each OCR file, and the method are in the source record. Rights: Public domain (US)
The Supplement is not a Project Gutenberg text. It is Bibliotheca Catholica’s transcription of public-domain scans at the Internet Archive; obvious OCR errors were corrected, and passages marked “OCR uncertain” could not be confidently corrected, so check them against the linked scan page.
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.
Obj. 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.
Obj. 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.
Obj. 4: Further, this impediment is not mentioned among those enumerated above (Q. L.). 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 connexion 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 connexion 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.
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Also online, linked only: the same translation at New Advent and the Latin text at Corpus Thomisticum. Neither is copied here.