Among the debates that became prominent during the 19th century American Presbyterian (and Reformed) churches was the debate over what restrictions the Bible taught concerning relations of affinity and consanguinity within marriage.
The Westminster Confession (24:4., 1646) states:
"IV. Marriage ought not to be within the degrees of consanguinity or affinity forbidden in the Word; nor can such incestuous marriages ever be made lawful by any law of man, or consent of parties, so as those persons may live together, as man and wife. The man may not marry any of his wife's kindred nearer in blood than he may of his own, nor the woman of her husband's kindred nearer in blood than of her own."
The annotations of the Dutch Statenvertaling Bible, authorized by the Great Synod of Dort, on Leviticus 18:16, 18 also show the view of the Dutch Reformed Church, which was consistent with Westminster.
However, this particular understanding of the Levitical laws regarding marriage (the Westminster Assembly's proof-texts include New Testament passages as well as those from the book of Leviticus), has been often challenged in the years since.
"When Samuel Miller was examined for licensure and ordination he took exception to the affinity sentence [WCF 24:4], but later in his ministry his view changed and he became convinced of the accuracy of the sentence." (Barry Waugh, The History of a Confessional Sentence: The Events Leading up to the Inclusion o f the Affinity Sentence in the Westminster Confession o f Faith, Chapter 24, Section 4, and the Judicial History Contributing to its Removal in the American Presbyterian Church, 2002 Ph.D., p. 274) Dr. Waugh notes the inclusion of two particular volumes contained in Samuel Miller's library after his death, which shed light on Miller's thinking on this matter. In a footnote on p. 111 of Dr. Waugh's most helpful dissertation, he writes: "Dr. Miller maintained some interest in the issues of marital affinity because the inventory of his library following his death revealed two near-kin titles. One book is Janeway’s, Unlawful Marriage, 1843, and the other is an 1816 work described as a Dissertation on Marrying a Wife's Sister, which may be Livingston’s A Dissertation on the Marriage of a Man with His Sister in Law (New Brunswick: Deare & Myer, 1816). See: Samuel Miller, “Catalogue, As Found at his Death,” PTSEM 1:3, pp. 2, 35."
Jacob Jones Janeway (1774-1858), in fact, studied under John Henry Livingston (1746-1825), who was not strictly speaking a Presbyterian, but was a leader of the Dutch Reformed Church in America. Janeway notes in the introduction to his book that the Dutch Reformed Church rescinded the rule against marriage to a deceased wife's sister in 1842. Both men opposed the tide that was overtaking both the American Presbyterian Church and the Dutch Reformed Church in America. Charles Hodge (1797-1878) in his Systematic Theology, Vol. 3, pp. 413ff, and elsewhere, also argued likewise.
Nevertheless, in 1886, the Presbyterian Church in the United States officially removed the affinity sentence from WCF 24:4. This revision was retained in the 1936 WCF by the Orthodox Presbyterian Church (OPC) and, later, by the Presbyterian Church in America (PCA).
The works touching on this topic highlighted above can be read online at Log College Press. They provide a window not only into Samuel Miller's library, but also into a controversy that occupied many 19th century Presbyterian General Assemblies, and, though decided in certain branches of the Reformed and Presbyterian churches, continues to reverberate today.