Breaking of marriage after engagement

Dear Sir/Ma'am, I need help. My marriage is fixed by parents. My engagement and Tilak ceremony was conducted same day in the presence of families and relatives of both sides one month ago on 24th Aug,18. Marriage date is fixed as 27th Jan,19. My fiancee is in a serious relationship with a guy and her father also know this but was forcefully manager her to marry with me. My fiancee is cheating me and being in touch and meet with that guy and continue to talk with him each and every day 3-6 hrs (i.e. her morning's 1st call to late nights last call is only with him). I cought her and managed to get her statement of her call details of last 6 months from telecom service provider. I have shocked after seeing the details of her call because each calls is only from her side to her lover and daily 3-6 hrs talk. This is realy too much. I have gave the statement of her calls details to her father and told him to break marriage. That time he accepted that he also know the facts (i have audio recording with me). But now he threatened me and my family not to break marriage otherwise he will take legal actions i.e. dowry harassment, cheating case and many other against me and my family members. He has also gave me an option to settle the matter with Rs.11 lakh. I am not guilty in this matter in any way then why we suffer. Please suggest me what to do. Is her father realy charged us in dowry harassment? what to her daughter who is cheating me since starting? Request you to please help me out. Thank you so much in advance.