The impediment of affinity
Objection 1. It would seem that in such a suit one ought not to proceed by hearing witnesses, in the same way as in other suits where any witnesses may be called provided they be unexceptionable. But here strangers are not admitted, although they be unexceptionable. Therefore, etc.
Objection 2. Further, witnesses who are suspected of private hatred or love are debarred from giving evidence. Now relatives are especially open to suspicion of love for one party, and hatred for the other. Therefore their evidence should not be taken.
Objection 3. Further, marriage is a more favorable suit than those others in which purely corporeal questions are at stake. Now in these the same person cannot be both accuser and witness. Neither therefore can this be in a matrimonial suit; and so it would appear that it is not right to proceed by hearing witnesses in a suit of this kind.
On the contrary, Witnesses are called in a suit in order to give the judge evidence concerning matters of doubt. Now evidence should be afforded the judge in this suit as in other suits, since he must not pronounce a hasty judgment on what is not proven. Therefore here as in other lawsuits witnesses should be called.
I answer that, In this kind of lawsuit as in others, truth must be unveiled by witnesses: yet, as the lawyers say, there are many things peculiar to this suit; namely that "the same person can be accuser and witness; that evidence is not taken 'on oath of calumny,' since it is a quasi-spiritual lawsuit; that relatives are allowed as witnesses; that the juridical order is not perfectly observed, since if the denunciation has been made, and the suit is uncontested, the defendant may be excommunicated if contumacious; that hearsay evidence is admitted; and that witnesses may be called after the publication of the names of the witnesses." All this is in order to prevent the sin that may occur in such a union (cap. Quoties aliqui; cap. Super eo, De test. et attest.; cap. Literas, De juram. calumn.).
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)