From the opening paragraph to the section entitled “Concise and Accurate
Instructions Concerning Marriages”: Since the sacred Canons, Apostolic, Conciliar, and Patristic, in
speaking in various parts about lawful and unlawful marriages (e.g., Ap. c.
XIX; cc. III and LIV of the 6th; c. II of Neocaesarea; cc. XXIII, XXVII,
XXVIII, LXVIII, LXXVI, LXXVII, LXXVIII of Basil; c. XI of Tim.; and cc. V, XIII
or Theophilus), requires a knowledge of marriages to be combined with them,
therefore and on this account we have judged it reasonable, after interpreting
the sacred Canons, to insert in a special place within the volume, for a
clearer comprehension on the part of the more unlearned, a concise and at the
same time accurate set of instructions regarding marriages allowed by the laws
as well as those prohibited, in view of the fact that such instructions are
needed by all, persons in general, but especially by the holy Prelates and
Spiritual Fathers and Priests, who have an obligation to examine into all these
matters....
This relationship results when one sponsors a child at the ceremony of Holy
Baptism. For the man who undertakes this sponsorship is making the child in
question his spiritual son or daughter, as the case may be; accordingly, he in
fact becomes a closer and more intimate relative and father of the child than
is its carnal father, because just as much as the spirit is higher than the
body the relationship of the spirit is higher than that of the flesh. Hence in
accordance with this ratio of intimacy c. LIII of the 6th Ecumenical Council
asserts that relationship in respect of spirituality is greater than any relationship
in respect of carnality. Some persons, however, in reading the present
Canon failed to understand it in this vein, as respecting the quality of
intimacy, but took it to refer to quantity of degrees. Wherefore they even
extended the relationship due to baptism to the seventh degree. Others, again,
even prohibited the eighth degree, which is more than holds with respect to
blood relationship. Though this may not please most men, as Blastaris says
(alphabetical section Beta), yet it pleases them to have only those
persons prohibited who are prohibited by the law. But the law prohibited, not
collateral relatives—brothers and sisters, that is to say, of a godfather and
of a godson, but only those in the direct line; and even these not to the eighth
degree, but only to the third. In other words, the law simply prohibited a
godfather from marrying his goddaughter, or her mother or her daughter, but
neither may the son of the godfather take to wife any one of these three. So:
Prohibited Marriages
1.—A godfather (or his carnal son) may not marry his goddaughter, i.e., any
girl that he has baptized; because she is a spiritual daughter of his and of
the first degree in relation to him, while in relation to his son she is a
spiritual sister, and consequently of the second degree.
2.—Nor her mother, nor her daughter; because they are of the second degree.
3.—None of the godfather’s children may marry the mother of their father’s
godchild, since they are considered nephews and nieces in relation to her, and
are of the third degree.
4.—Nor may any child marry a daughter of his father’s goddaughter (or
conversely), because he is considered a spiritual uncle in relation to her, and
consequently is of the third degree.
Unprohibited
Marriages
1.—A son of the godfather may marry the sister of her godfather’s son,
according to Blastaris, or vice versa, the brother of a godfather’s godson may
marry the godsister of his brother.
2.—The carnal son of a godfather may marry the carnal sister of the godson;
for according to most authorities she is of no degree, owing to her
relationship being collateral.
3.—Likewise the brother of a godfather may marry the sister of his godson;
and conversely the brother of the godson may marry the sister of the godfather.
4.—Two spiritual brothers or spiritual sisters, or a spiritual brother and
a spiritual sister, having the same godfather, may marry two carnal sisters.
Prohibited Marriages
1.—If perchance two children, one male and the other female, happen to be
baptized by one and the same godfather, they may not marry each other, because
they are spiritually brother and sister, according to most holy St. Sisinnius,
and are of the second degree.
2.—A man may not marry the widow of his spiritual brother, because he too
is considered to be a brother of hers, owing to his brother’s having contracted
a relationship with her making the two of them one flesh, and therefore she is
of the second degree in relation to him.
3.—Likewise if the husband baptizes one child, and his wife another, these
children may not intermarry; because it is plain that they were baptized by the
same sponsor, owing to the fact that a married couple is accounted one flesh,
notwithstanding that the godfather and the godmother are not one and the same
person.
4.—Likewise a son-in-law may not marry the goddaughter of his fatherin-
law, since she is considered a sister of his dead wife. One and the same man
can never marry two carnal or spiritual sisters.
Unprohibited or Doubtful Marriages
1. But if the father-in-law baptize one child, and the son-in-law another,
these children may intermarry, because a father-in-law is related to his
son-in-law collaterally (a latere), as some say. Yet a father-in-law in
relation to his son-in-law is considered to be related to him lineally and not
collaterally, owing to the fact that his son-in-law is united with his daughter
into one flesh. Hence such a marriage is doubtful and worth discussing.

Source: http://orthodoxinfo.com/praxis/relationship_due_to_baptism.aspx
CONVERSATION