In a legal sense adoption is an act by which a person, with the cooperation of the public authority, takes as his own the child of another.
Under Roman law legal relationship was established, based on the natural relationship, and it was a bar to marriage.
Its degrees were: civil fatherhood, between the adopter and the adopted and the latter's legitimate children; civil brotherhood, between the adopted and the legitimate children of the adopter; affinity, arising from the tie of adoption between the adopted and the adopter's wife, and between the adopter and the adopted's wife.
The Church, receiving this law as her own, recognized adoption as a diriment impediment of marriage.
The modification of the Roman law by the compilation of new codes led to discussions as to what extent this diriment impediment of legal relationship still exists in the eyes of the Church, and the principle was laid down by Benedict XIV that, wherever the elements of the Roman law are retained in the new codes, the Church recognizes this relationship as a diriment impediment.
In Great Britain and the United States legal adoption in the sense of the Roman law does not exist.
In the United States it is regulated by State statutes and is generally accomplished by mutual obligations assumed in the manner prescribed by law.
Adoption by private authority or under private arrangements is not recoguized by the Church as productive of this legal relationship.
Hence, in the United States, adoption is not generally a diriment impediment of marriage, nor in the eyes of the Church in any way preventive of it.
New Catholic Dictionary