Mortal sin has three requirements: grave matter, full knowledge, and full and free consent of the will. If any of those requirements are missing, there is no mortal sin. Barring the rare spiritual gift of reading souls, the most that someone can know about others is the actions they commit and whether those actions constitute grave matter. Most people cannot know whether another person has the requisite knowledge and consent for mortal sin. That is why it is not usually a good idea to presume to know whether someone else is in mortal sin and therefore may not receive Communion.
While it is good to pray for others’ conversion and to offer information when appropriate, at Communion time it is more prudent to examine our own consciences and determine our own worthiness to receive Communion. If you are worried for your parents’ spiritual welfare, one possibility might be to offer up your Communion for the intention of your parents’ conversion.
