Chapter XXIV. Of Marriage, and Divorce

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 final sentence was removed out of greater deference to the civil laws of the State.
In the end, this was a matter of interpretation, in keeping with the historic stream of the Church.
One of the great merits of the Westminster Confession of Faith is its close adherence to historic Christianity.
Present-day Protestantism has drifted far from historic Christianity, especially since the 19th century.