A girl whom I met 1 month ago is threatening me!

Dear Sir, I would like to inform you that I met a girl online named Manashi Das aged about 35 on facebook a month ago, a divorcee. Her sister named Poulomi Das, aged 22 used to chat with me using her elder sister's profile. She sent me a request. Me & poulomi spoke to each other & got close over phone only. She sometimes provoked me. We shared some close pictures bu I used to delete those after seeing. I made some talks regarding marriage & all. But I was never serious about her. I in fact never met her personally neither I got involved in any physical relation. I am in a relationship with a girl named Pallabi Dutta since 2014,June,04 & we are going to marry each other. But this Poulomi along with her elder sister manashi are continuously threatening me, that they are not going to stay me happy. They will beat me & will go to Police. I tried to explain them hundreds of times but they are saying me not to marry Pallabi else they are going to ruin my life & career. I'm working in a private organisation in Kolkata. My family's earning source in only me. My dad is a retired person & mom is a house wife. I need your help sir. Am I in a big trouble. Will I be put behind the bars?? Please suggest me what to do legally. I once again telling you that I have never personally met the girl Poulomi. We had spoken on facebook & on messages. Please help me Sir.