I commonly encounter this as an adult education catechist: two baptized catholics married by civil ceremony, without previous marriages. Can they receive Holy Communion before convalidation? If they are preparing to receive the sacrament of Confirmation, should they have their marriage convalidated first?
To not be validly married means that if they have sexual relations with someone they are not married to, they are guilty of the sin of fornication.
As long as they are living as husband and wife without being validly married to each other, they may not receive Holy Communion. If they live as brother and sister, they may.
Fr. Vincent Serpa, O.P.