my widow mother is in relation with other man help me

About me :-I am 23 years old, right now I'm on my degree(engineering, out of my state). I have one elder sister (25yrs), she got job and got married last month(currently in my state). My Father died on 28th May, 2015. The last day of my end semester exam. Things change after my Father died, my mom is having affairs with a married man from my village itself, due to this I and my Sister are in stress.... Every persons from my village stares at them, which we don't feel bad by seeing all this. Yesterday my Sister called me and told me that' I cut his bike and now he told me that I will cut you into pieces'. Please help me what to do now? Shall I file FIR to the police if not what steps should I take.