ANSWER:
It is up to the bishop to decide based on the nature of kinship created by the fact that two people share a godparent. The Church understands that this bond brings someone "into the family," which is why there are canons that consider this kinship relevant to marriage among relatives. In this case, a letter should be send to the bishop with all the details since he is the one that will make that determination. In the case you describe, it would seem that having the same sponsor as adult converts would not be considered an impediment to marriage. |