In order to be morally culpable for a mortal sin 3 conditions must be present (CCC 1857):
- The action must be gravely wrong
- The person must know it is gravely wrong
- The person must freely choose the action
If someone doesn’t understand they have also committed other mortal sins then condition #2 is not present and the person is not gravely culpable (though they may still bear some moral culpability).