Divorce is a civil declaration that the marriage is no longer. The Civil authorities have no ability to declare anything about a Sacrament.
An Annulment is a declaration that for some reason, (which is determined by examining the marriage at an ecclesiastical court) the sacrament never took place. To give an extreme example, if there was a “shotgun” marriage, and the couple lived as if they where really married, later when the threat no longer existed the party forced into the marriage came forward about being forced against their will, the ecclesiastical court would declare formally that the marriage (sacramentally) never existed.
Another causes for an invalid marriage include, such things as a man who is homosexual and tries to hide it by asking a woman to marry him, he withholds the fact of his sexuality to her, and enters into what looks to everyone as a real marriage. Because he has withheld this and entered into a sham marriage, it would be held to be invalid.
There are quite a few reasons why a marriage may not be valid (sacramentally) from the beginning, usually the reason for the annulment is not disclosed to the public, in order to protect the parties involved from embarrassment. They themselves know the grounds for the annulment, but the public may not know unless the person tells them.
In the first case in this thread, if a person gets a civil divorce, and does not attempt to marry (without an annulment) he or she may still approach the Sacraments, if they attempt to marry without an annulment they cannot receive the Sacraments. This is to protect the marriage, (if one really did exist in the first place. So unles there was another reason why the aunt could not receive the sacraments, (if she too had remarried without an annulment) then she would be free to approach the altar, and receive communion.