Ex gf threatening me.

I met a girl in July. She was about to get admission in Bangalore nursing college. I advised her to stay in hometown as I know couple of institutions. She and her mother decided to get admission in that institute rather Bangalore and advance payment was given by them. We gradually grown a likeness and one day we had sex at my home. Consensual but no false promise of marriage. Thereafter things started to get bitter. She was too immature and forcing me to talk to her always. Meanwhile her mother called me and asked what's going on between us. Then one day we three met and she asked me if she stays at hometown I have to marry her. If she doesn't want, I have to pay 1.5 lakh for her admission in Bangalore which she will return,as I "brainwashed" her. Otherwise she will tell her mother we had sex. Then i talked to my parents and after some days I told her it is not possible to give her that money and since then she and her mother started blackmailing me. She even took some pics of our intimate scene which I didn't know. She told me she will come to my home with the pic if I don't talk to her. Her mother accused me that I used her lady. Now she has got her advanced payment back from this institute and ready to go to Bangalore. But I need to keep a relationship with her and later marry her. Me, my mom, she and her mother discussed the thing in a place that it is not possible for me to continue this relationship. She threatens suicide now. Now she is telling she was helpless that's why she asked the money. They are trying to prove that I just used her and I am forced to marry her. If I don't receive her call they will come to my home and workplace. I am terribly afraid. What should I do? I don't want this family. What if they register a false rape case? Please help me..