Can I cancel my marriage as the boy is blackmailing me ???

I was in relationship with my boy friend and as his parents haven't agreed for our marriage I agreed for a marriage which my parents had searched for me.Now the problem is that he is a relative of ours and happens to be my brother-in-law and he knew about my past relationship as a family member.when i agreed to marry him he didn't say me anything he was quite good but after the marriage date was settled he started showing his true nature .He said me that he has that he started black mailing me with the call records and messages of me and my boyfriend.he is saying that he inquired about us and got my call records for the past few months with the help of a police officer.When i asked him he simply said your father promised me that he would get me married to you but as you are in a relation with another person and cheated me i have all rights to get your call records. He even threatened me to show all those messages and call records in which we had sex talks and break the marriage .he tried to harass me even sexually. He would always abuse my parents threaten me saying that he would give a police complaint against me with all those chats saying i had relation with him . now i have to decided to cancel the marriage as he would talk very bad of me like are u a virgin ?? with how many people did u sleep ...u people did'n look me well when i came to your home are second hand and he would be monitoring me all day long so that i don't talk to my ex. (to be honest after I agreed for marriage i didn't speak to my ex even once)He always keeps saying me oh remembering your ex !!! I know that he really got my call records which is a crime he himself said me many things which were our personal that me n my ex spoke of. I clearly know that he has tracked and tapped my calls but i don't have proofs as i cancelled my marriage because of his words and actions which really hurt me very bad. He is threatening me even now saying he will proceed legally i m really afraid could anyone please help me can he put a case against me for canceling marriage and if yes what are the precautionary problems i need to take?? Please do suggest me.