I don’t recall ever hearing that believing the lie of Satan was Eve’s sin (but I could be wrong). Her sin was in disobeying God. She knew what God wanted, yet chose to disobey him. Her sin was disobedience (which also carries an element of pride int it).
Grave matter may be the matter of mortal or venial sin. Formal sin is wrong and known by the actor to be wrong, so there will be culpability when freely commited.
1790 A human being must always obey the certain judgment of his conscience. If he were deliberately to act against it, he would condemn himself. Yet it can happen that moral conscience remains in ignorance and makes erroneous judgments about acts to be performed or already committed.
1791 This ignorance can often be imputed to personal responsibility. This is the case when a man "takes little trouble to find out what is true and good, or when conscience is by degrees almost blinded through the habit of committing sin."59 In such cases, the person is culpable for the evil he commits.