Objection 1. It would seem that affinity in itself admits of degrees. For any kind of propinquity can itself be the subject of degrees. Now affinity is a kind of propinquity. Therefore it has degrees in itself apart from the degrees of consanguinity by which it is caused.
Objection 2. Further, it is stated in the text (Sent. iv, D, 41) that the child of a second marriage could not take a consort from within the degrees of affinity of the first husband. But this would not be the case unless the son of an affine were also affine. Therefore affinity like consanguinity admits itself of degrees.
On the contrary, Affinity is caused by consanguinity. Therefore all the degrees of affinity are caused by the degrees of consanguinity: and so it has no degrees of itself.
I answer that, A thing does not of itself admit of being divided except in reference to something belonging to it by reason of its genus: thus animal is divided into rational and irrational and not into white and black. Now carnal procreation has a direct relation to consanguinity, because the tie of consanguinity is immediately contracted through it; whereas it has no relation to affinity except through consanguinity which is the latter's cause. Wherefore since the degrees of relationship are distinguished in reference to carnal procreation, the distinction of degrees is directly and immediately referable to consanguinity, and to affinity through consanguinity. Hence the general rule in seeking the degrees of affinity is that in whatever degree of consanguinity I am related to the husband, in that same degree of affinity I am related to the wife.
Reply to Objection 1. The degrees in propinquity of relationship can only be taken in reference to ascent and descent of propagation, to which affinity is compared only through consanguinity. Wherefore affinity has no direct degrees, but derives them according to the degrees of consanguinity.
Reply to Objection 2. Formerly it used to be said that the son of my affine by a second marriage was affine to me, not directly but accidentally as it were: wherefore he was forbidden to marry on account of the justice of public honesty rather than affinity. And for this reason this prohibition is now revoked.
Source: Summa Theologica (New Advent)