(b) It is committed with a relative, that is, with a person, male or female, who is near to one by the tie of common ancestry (blood relationship, kinship, consanguinity), or of marriage to one’s kin (marriage relationship, affinity), or of sacramental administration (spiritual relationship), or of adoption (legal relationship). Alias species cognationis non pertinent ad incestum, sed novam aliquam malitiam possunt tribuere; v.g., si partes sunt parochus et parochiana, confessarius et poenitens, habetur scandalum, seductio.
(c) The relationship is within the canonical degrees. Thus, marriage between blood relatives is forbidden in all degrees of the direct line (e.g., as to all female ancestry and posterity of a man) and in the first three degrees of the collateral line, which includes, for a man, his sisters, nieces, grandnieces, aunts, first and second cousins, grand aunts and their daughters and granddaughters. Marriage between those who are relatives-in-law is forbidden in all degrees of the direct line (e.g., as to wife’s mother, daughter, etc.) and in the first two degrees of the collateral line (e.g., wife’s sister, first cousin, aunt or niece). Spiritual relationship which is impedient of marriage exists between a person baptized and his baptizer, and also between the god-child and the god-parent in baptism. Legal relationship exists between the adopter and the adopted, when and as the civil law makes it a bar to marriage.
(d) Incest is committed within the forbidden degrees, and hence if a dispensation from an impediment of relationship had been granted to parties about to marry, a sin between them would not be incestuous.
2532. Incest as a Distinct Species of Sin.—(a) There is a specific distinction between incest and other forms of lust, since incest violates not only purity, but also the piety and respect due each other by those who are so closely related as to be unable to contract a lawful marriage. Nature itself abhors this sin; for, apart from the exceptional cases in which a dispensation is given, even lawful marriage with near relatives would be an incentive to many sins before marriage and would prevent the widening circle of friendships between mankind which marriage with non-relatives produces, and would cause a physical and mental enfeeblement of the race. In Scripture incest is spoken of with peculiar horror as a nefarious deed deserving of death (Lev., xx. 11 sqq.), and as an act unworthy even of pagans (I Cor., v. 1 sqq.).
(b) There are three distinct sub-species of incest, namely, natural incest (between kin by blood or marriage), spiritual incest (between the baptized and his baptizer or god-parent), and legal incest (between persons who are kin in virtue of 8, marriage-impeding adoption). The first violates piety due to natural origin, the second that due to spiritual origin, and the third that due to legal origin. And in each species the nearer the relationship, the greater the sin (e.g., incest with a sister-in-law is less than that with a sister, incest with a sister is less than that with a mother).
2533. Carnal Sacrilege.—Carnal sacrilege is the violation by an act of impurity of the sacredness of a person, place or thing.
(a) It is a violation of sacredness, and thus it is a special sin, adding irreligion to lust (see 2308 sqq.).
(b) It is an act of impurity, internal or external, consummated or non-consummated. The impurity, however, must be so related to that which is sacred as to treat its sanctity with injury or contempt (formal disrespect), and there is no sacrilege if the impurity is associated with something holy in such a way as not to show any notable irreverence (material disrespect).
(c) Its first species is personal sacrilege, and it is committed by a sacred person (see 2309) when he is impure internally or externally, or by a non-sacred person when in desire or act he commits impurity with a sacred person. If two sacred persons sin together, there is a double sacrilege, which multiplies the sin.
(d) Its second species is local sacrilege, and is committed when an impure act is done in a sacred place (2311) in such a way as to show formal disrespect. Hence, consummated acts done in a church are sacrilegious, and the same is probably true of non-consummated acts, at least if they are of an enormous kind (e.g., a lascivious dance), and even of internal desires to sin in the sacred place. But impure thoughts or passing glances of prurient curiosity in a church are not sacrilegious.