Objection 1. It would seem that a relationship of this kind is contracted only between the adopting father and the adopted child. For it would seem that it ought above all to be contracted between the adopting father and the natural mother of the adopted, as happens in spiritual relationship. Yet there is no legal relationship between them. Therefore it is not contracted between any other persons besides the adopter and adopted.
Objection 2. Further, the relationship that impedes marriage is a perpetual impediment. But there is not a perpetual impediment between the adopted son and the naturally begotten daughter of the adopted; because when the adoption terminates at the death of the adopter, or when the adopted comes of age, the latter can marry her. Therefore he was not related to her in such a way as to prevent him from marrying her.
Objection 3. Further, spiritual relationship passes to no person incapable of being a god-parent; wherefore it does not pass to one who is not baptized. Now a woman cannot adopt, as stated above (1, ad 2). Therefore legal relationship does not pass from husband to wife.
Objection 4. Further, spiritual relationship is stronger than legal. But spiritual relationship does not pass to a grandchild. Neither, therefore, does legal relationship.
On the contrary, Legal relationship is more in agreement with carnal union or procreation than spiritual relationship is. But spiritual relationship passes to another person. Therefore legal relationship does so also.
Further, the same is proved by the authorities quoted in the text (Sent. iv, D, 42).
I answer that, Legal relationship is of three kinds. The first is in the descending order as it were, and is contracted between the adoptive father and the adopted child, the latter's child grandchild and so on; the second is between the adopted child and the naturally begotten child; the third is like a kind of affinity, and is between the adoptive father and the wife of the adopted son, or contrariwise between the adopted son and the wife of the adoptive father. Accordingly the first and third relationships are perpetual impediments to marriage: but the second is not, but only so long as the adopted person remains under the power of the adoptive father, wherefore when the father dies or when the child comes of age, they can be married.
Reply to Objection 1. By spiritual generation the son is not withdrawn from the father's power, as in the case of adoption, so that the godson remains the son of both at the same time, whereas the adopted son does not. Hence no relationship is contracted between the adoptive father and the natural mother or father, as was the case in spiritual relationship.
Reply to Objection 2. Legal relationship is an impediment to marriage on account of the parties dwelling together: hence when the need for dwelling together ceases, it is not unreasonable that the aforesaid tie cease, for instance when he ceases to be under the power of the same father. But the adoptive father and his wife always retain a certain authority over their adopted son and his wife, wherefore the tie between them remains.
Reply to Objection 3. Even a woman can adopt by permission of the sovereign, wherefore legal relationship passes also to her. Moreover the reason why spiritual relationship does not pass to a non-baptized person is not because such a person cannot be a god-parent but because he is not a fit subject of spirituality.
Reply to Objection 4. By spiritual relationship the son is not placed under the power and care of the godfather, as in legal relationship: because it is necessary that whatever is in the son's power pass under the power of the adoptive father. Wherefore if a father be adopted the children and grandchildren who are in the power of the person adopted are adopted also.
Source: Summa Theologica (New Advent)