What to do in case of of false criminal allegations on my mother

Relations between my mother and tai were estranged after my fathers death due to some family problems. . After that she has sent a fake letter to her office(with false name) and imposed false allegations on her.my mother is a clean government employee, so she was given a clean chit immediately. furiously my mother has written the letter to my cousin sister husband about her previous affairs with a boy. When I came to know about this I and all my maasis scolded her for her childish behaviour then . After that she got so furious that she started threatening her to lose her job. Ultimately ,we were called by the police where police scolded my mother on 19th January,2017 and leave her after giving her warning(as it was simple case of tensions between two sister in laws). But on 4th February ,police has registed an fir where my tai has accused my mother of abusing her and getting physical on her on the same day of 19th January,2017.Sir, it is a baselessfir she wasduty that day..