Objection 1. It would seem that spiritual relationship does not pass to the godfather's carnal children. For no degrees are assigned to spiritual relationship. Yet there would be degrees if it passed from father to son, since the person begotten involves a change of degree, as stated above (Question 55, Article 5). Therefore it does not pass to the godfather's carnal sons.
Objection 2. Further, father and son are related in the same degree as brother and brother. If therefore spiritual relationship passes from father to son, it will equally pass from brother to brother: and this is false.
On the contrary, This is proved by authority quoted in the text (Sent. iv, D, 42).
I answer that, A son is something of his father and not conversely (Ethic. viii, 12): wherefore spiritual relationship passes from father to his carnal son and not conversely. Thus it is clear that there are three spiritual relationships: one called spiritual fatherhood between godfather and godchild; another called co-paternity between the godparent and carnal parent of the same person; and the third is called spiritual brotherhood, between godchild and the carnal children of the same parent. Each of these hinders the contracting of marriage and voids the contract.
Reply to Objection 1. The addition of a person by carnal generation entails a degree with regard to a person connected by the same kind of relationship, but not with regard to one connected by another kind of relationship. Thus a son is connected with his father's wife in the same degree as his father, but by another kind of relationship. Now spiritual relationship differs in kind from carnal. Wherefore a godson is not related to his godfather's carnal son in the same degree as the latter's father is related to him, through whom the spiritual relationship is contracted. Consequently it does not follow that spiritual relationship admits of degrees.
Reply to Objection 2. A man is not part of his brother as a son is of his father. But a wife is part of her husband, since she is made one with him in body. Consequently the relationship does not pass from brother to brother, whether the brother be born before or after spiritual brotherhood.
Source: Summa Theologica (New Advent)