Problems due to improper marriage registration

My sister did register marriage with his friend during last year of college,after when she returned to home ,we found that she had registration certificate,when we came to know about her marriage details,she left home and went to live with him,when we got details about registered marrige we found that ,she did register marriage in a district which is no where related to ours,and they gave marriage invitation and certificate issued by temple as proof,but we found both are fake and now she feels for her mistake and want to come back to us,plz advise on this,can we challenge registar about marriage,can it be made null/void.note-even the witness say the boy forced them and emotionally blackmailed them to put witness certificate,plz advise on this .