My girlfriend's brother is threatening me that he will file an FIR against me

I came into relationship with a girl in my 3rd semester . I am 27 years old and she is 26. In the very beginning we discussed that our future will be difficult as we both are from different caste, so it will also affect our decision to get married. We go to restaurants, movies, hold hands, kissed but never get fully physical, and I also never force her for anything. After the lockdown imposed she went her hometown and same for me, and we chat, talked on calls and video calls till July 2020, but after that we slowly stopped talking, after that we greet each other formally only. I also got busy in competitive exams as I became more focused towards job, so I gave her less attention, this was one of the reasons, I guess. We chatted formally till 3rd week of may 2021. (Our friends never know about us, as we decide not to disclose it till we get some surety of where we are going). In the same time I heard from our mutual friends that she was mentally disturb and also his family came to know about that she had a boyfriend. Her brother was enquiring why is she behaving like this suddenly. I came to know about this, so I dropped a message, to call me whenever she is free. I received a call from her what's app. Her brother was on the other side and started abusing me a lot, threatening me that he is going to file an FIR against me, that she has been behaving like this because of me, and warned me not to msg or call her ever again. I didn't told this to anyone, but discuss with friends to find out how is she, and what was happening at her home. My friends told me that whenever they talk with her, she acts normal. Days passed by and today on 26th July 2021 I got a call from my friend, asking was there anything between the two of us, I told everything. My friend told me that she received a call from my gf's brother, and he told her that he filled a complaint against me, and he also said that we forced her to go to my brother's marriage ceremony which held in March 2020, but there was nothing like that. Our whole group went to the marriage. Her brother has also taken her phone, as I was told by my friend. And he occasionally call my female friends and enquire. What should I do ? I am afraid for myself, my family and also for my friends, they will all get involve in this because of me...