Can We Declare Marriage Illegal

Sir, My sister was kidnapped six month ago and she was married forcibly.Fir was against unknown but after investigation 7 culprit person was revealed.My sister has also given statement under threaten and pressure in crpc 164 according to their wish. Now my family don't want to face court and police station too much because every one is supporting culprit as we are not rich like them.And also other party is threatening that he would not let my sister marry with any one at any cost because he has nikahnama with my sister's sign(which was taken forcibly).My sister was engaged with some one and he is still ready to marry my sister but his demand is every kind of clearance so that other party would not torture him in future(He is afraid of facing any case due to marriage in future).My family just want to get rid off this .Is there any law by which court can declare marriage null and void or i just want to know how much time it will take to complete the procedure as we want my sister marriage off as soon as possible. Could you please advice me any section in law which is useful in my case or suggest me needful procedure to tackle this situation. Thanks in Advance