A girl is threatening my brother

Hello Sir, My name is Arun, I'm writing on behalf of my brother who is now facing an issue with a girl. My brother is working in Chennai in an IT firm, he got engaged to a girl last December in Kerala, arranged by my parents to his own will. 2 days before a girl sent message to the girl who my brother going to marry and to her brother through Facebook messenger. That girl has sent msg to her in an indecent way that, my brother is a cheater and they had relationship of over 8 years, after my brother got engaged he blocked her and no msgs, and no calls. And also she says in msg that my brother had all sex chat with her, and his only intention is to play with girls life and he will do the same thing with the new girl also. No she is asking the marriage date, so that she can give the proof of everything. Now the new girl is very much upset and sad and broke, her total family is sad, she says my life has gone. We can't handle her. My parents too, my mother’s health condition is already worse, hearing this she's in more pain. So I checked with my brother, he says that he knows that girl through Facebook and had chat and also called sometimes, but had no relationship with her, she proposed to him, but he never had any relationship as she says in the message. i know abt my brother as he even don’t speak even to our relatives and a moody type. He said that that girl is proposing him and he blocked her, but she creates other profile and sending message to him. Now Sir, please help us how to handle this situation… Can I talk with that girl and make her understand what she really needs. Now if this marriage didn’t happen then we will face lot of issues, we will be in great shame in front of relatives and friends and the life of this new girl also is a big question mark. So please help us to solve this issue.