Ex is threatning for marriage

I have met a girl through common family relative for marriage but after talking to her for a while I understand that our marriage cant be sustained and we both decided to end it there. Still she feels that something can be happened After sometime I approaches her to have a second thought again becoz she was ready to do necessary changes required. We both started talking to each other. But again day by day things where getting toxic so I told her that we cant get married and she agreed and we stopped talking again There is no physical involvement till now But after 0ne month she again started calling me and told me to give it a try and lets get into relationship and later think of marriage if happens. I clearly said that we cant get marriad but she convinced me for relationship and then we had some physical moments She was still hopefull that things will change and get better but I was very clear and have always made her clear that we cant get marriad and during this time she met few other guys and had physical relation as well Now I am looking forward to get married but her brother is threatning me for FIR Please suggest