A questioner in another thread asked about what her sister needed to do to be able to remarry. The answer was as follows:
Unless your sister attempts marriage again, she needs only to go to confession. The Church considers her still married to her 1st husband although civilly divorced.
To attempt marriage again, she will need to have her 1st, 3rd, and 4th marriages annulled - her 2nd marriage, if valid, was dissolved when her husband died.
My quesiotn is this: if only the first marriage can be sacramentally valid and the couple is considered to be married regardless of the decision of the courts, why is it necessary to receive an annulment for second, third, fourth, etc, marriages? Should not the annulment from the first be sufficient. Then, confession would be required for the subsequent marriages, as they would be situations of adultery in the eyes of the church, but not marriages.