I often think of that.
But unless the priest is keeping people under bedroom surveillance -- drones at the window, maybe? --- he has no idea whether they are living in sexual continence, as brother and sister, as they ought to do if they are not validly married.
He can tell them but -- like I said --- if they intend to keep on sinning, that's on them.
And of course, that goes for all of us.
And considering the wide scope of possible reasons why a marriage may be annulled, and an est. 400,000 marriages have been annulled since 1970 (http://articles.philly.com/1986-05-08/news/26049605_1_annulments-divorced-catholics-marriage), then how many RCs today are possibly in invalid marriages, even though canon law (1060) presumes all marriages are valid until proven invalid.
And as a SV site points out,
68% of annulments today [dated] are granted because of "defective consent," which involves at least one of the parties not having sufficient knowledge or maturity to know what was involved in marriage. The ingenuity of judges in confidently asserting that such knowledge or maturity was lacking is amazing. Vasoli says that it is done by substituting "junk psychology" for sound psychology and psychiatry. (www.mostholyfamilymonastery.com/28_Annulments.pdf)
Some of the conditions that can allow for annulment, from the Catholic Diocese of Arlington
Among the signs that might indicate reasons to investigate for an annulment are:
marriage that excluded at the time of the wedding the right to children, or to a permanent marriage, or to an exclusive commitment.
In addition, there are youthful marriages;
marriages of very short duration;
marriages marked by serious emotional, physical, or substance abuse;
deviant sexual practices;
profound and consistent irresponsibility and lack of commitment;
conditional consent to a marriage;
fraud or deceit to elicit spousal consent;
serious mental illness; or a previous bond of marriage.
- www.arlingtondiocese.org/tribunal/faq.php#Grounds
Rome also considers entering marriage with the intention of never having children to be a "grave wrong and more than likely grounds for an annulment."[McLachlan, P. "Sacrament of Holy Matrimony." http://www.catholicdoors.com/faq/qu164.htm] , while praying to a women who apparently went thru with a marriage intending to do just that,
► MATRIMONIAL CONSENT and annulment
Can. 1095 The following are incapable of contracting marriage:
1/ those who lack the sufficient use of reason;
2/ those who suffer from a grave defect of discretion of judgment concerning the essential matrimonial rights and duties mutually to be handed over and accepted;
3/ those who are not able to assume the essential obligations of marriage for causes of a psychic nature [all are judgment calls which can see varying verdicts].
List of diriment impediments to marriage
Age.[6] If the man is under 16 years of age, or the woman is under 14 years of age, then their marriage is invalid. This is an ecclesiastical impediment, and so does not apply to a marriage between two non-Catholics. However, note that in a marriage between a Catholic and a non-Catholic, the age limitation applies to the non-Catholic party as well.[7]
Physical capacity for consummation lacking [15]. Per Canon 1084 §3 "Without prejudice to the provisions of Canon 1098, sterility neither forbids nor invalidates a marriage." Both parties, however, must be physically capable of completed vaginal intercourse, wherein the man ejaculates "true semen" into the woman's vagina. (See [1] for details.)
To invalidate a marriage, the impotence must be perpetual (i.e., incurable) and antecedent to the marriage. The impotence can either be absolute or relative. This impediment is generally considered to derive from divine natural law, and so cannot be dispensed.[16] The reason behind this impediment is explained in the Summa Theologica:[17]
“In marriage there is a contract whereby one is bound to pay the other the marital debt: wherefore just as in other contracts, the bond is unfitting if a person bind himself to what he cannot give or do, so the marriage contract is unfitting, if it be made by one who cannot pay the marital debt.”
Previous marriage [18]. Previous marriages, whether conducted in the Catholic Church, in another church, or by the State. All previous attempts at marriage by both parties wishing to marry must be declared null prior to a wedding in the Catholic Church, without regard to the religion of the party previously married. Divine, absolute, temporary.
Disparity of cult [19]. A marriage between a Catholic and a non-baptized person is invalid, unless this impediment is dispensed by the local ordinary. Ecclesiastical, relative.
Sacred orders [20]. One of the parties has received sacred orders. Ecclesiastical, absolute, permanent (unless dispensed by the Apostolic See).
Perpetual vow of chastity [21]. One of the parties has made a public perpetual vow of chastity. Ecclesiastical, absolute, permanent (unless dispensed by the Apostolic See).
Abduction [22]. One of the parties, usually the woman, has been abducted with the view of contracting marriage.
Ecclesiastical,[citation needed] temporary.
Crimen [23]. One or both of the parties has brought about the death of a spouse with the view of entering marriage with each other. Ecclesiastical, relative, permanent (unless dispensed by the Apostolic See).
Consanguinity [24]. The parties are closely related by blood.
Ecclesiastical or divine, depending on the degree of relationship. Relative, permanent.
Affinity [25]. The parties are related by marriage in a prohibited degree. Ecclesiastical, relative, permanent.
Public propriety [26]. The parties are "related" by notorious concubinage. Ecclesiastical, relative, permanent.
Adoption [27]. The parties are related by adoption. Ecclesiastical, relative, permanent.
Spiritual relationship [28]. One of the parties is the godparent of the other. This no longer applies in the Latin Rite, but still applies in the Eastern Catholic Churches.
http://en.wikipedia.org/wiki/Canonical_impediment#List_of_diriment_impediments_to_marriage
. Wide interpretive provisions allow for saying the marriage never existed, and can see varying verdicts.
And then there is Pauline Privilege, according to http://www.catholicculture.org/culture/library/view.cfm?id=7272,
Pauline Privilege is the dissolution of a purely natural (not sacramental) marriage which had been contracted between two non-Christians, one of whom has since become a Christian. But if a Catholic marries an unbaptized;/non-Christian person is not a sacrament. The church says (based on a passage in Paul) that such a marriage can be dissolved for a grave reason, like if the unbaptized party makes it impossible for the Catholic to practice his faith.
The Pauline Privilege does not apply when a Christian has married a non-Christian. In those cases, a natural marriage exists and can be dissolved for a just cause, but by what is called the Petrine Privilege rather than by the Pauline Privilege. The Petrine Privilege is so-named because it is reserved to the Holy See, so only Rome can grant the Petrine Privilege.
Though i am hesitant to utterly disallow any extreme circumstances as possibly allowing grounds for annulment, yet in the Bible, although marriage as a commitment and social contract was generally understood, once a wife was taken — even foreign wives, or out lust, or even instead of the one contracted for, etc. — and the marriage was consummated, then such were considered to be married, and in no place are consummated marriages “annulled,” meaning they did not exist. Even concubines were wives. (Gn. 25:1; cf. 1Ch. 1:32; Gn. 30:4; cf. Gn. 35:22; 2Sam. 16:21, 22, cf. 2Sam. 20:3)