Objection 1. It would seem that the degrees of affinity do not extend in the same way as the degrees of consanguinity. For the tie of affinity is less strong than the tie of consanguinity, since affinity arises from consanguinity in diversity of species, as from an equivocal cause. Now the stronger the tie the longer it lasts. Therefore the tie of affinity does not last to the same number of degrees as consanguinity.
Objection 2. Further, human law should imitate Divine law. Now according to the Divine law certain degrees of consanguinity were forbidden, in which degrees affinity was not an impediment to marriage: as instanced in a brother's wife whom a man could marry although he could not marry her sister. Therefore now too the prohibition of affinity and consanguinity should not extend to the same degrees.
On the contrary, A woman is connected with me by affinity from the very fact that she is married to a blood-relation of mine. Therefore in whatever degree her husband is related to me by blood she is related to me in that same degree by affinity: and so the degrees of affinity should be reckoned in the same number as the degrees of consanguinity.
I answer that, Since the degrees of affinity are reckoned according to the degrees of consanguinity, the degrees of affinity must needs be the same in number as those of consanguinity. Nevertheless, affinity being a lesser tie than consanguinity, both formerly and now, a dispensation is more easily granted in the more remote degrees of affinity than in the remote degrees of consanguinity.
Reply to Objection 1. The fact that the tie of affinity is less than the tie of consanguinity causes a difference in the kind of relationship but not in the degrees. Hence this argument is not to the point.
Reply to Objection 2. A man could not take his deceased brother's wife except, in the case when the latter died without issue, in order to raise up seed to his brother. This was requisite at a time when religious worship was propagated by means of the propagation of the flesh, which is not the case now. Hence it is clear that he did not marry her in his own person as it were, but as supplying the place of his brother.
Source: Summa Theologica (New Advent)