Cheating and False allegations

My sister got married 20 years back in West Bengal and due to her vociferous nature and unable to mingle with people in a joint family she left with her husband within few months after marriage to NCR. She is very outrageous in nature and she has broken all relationships with her closed ones. Now her husband also left her because of her baseless allegations against him and daily fights over small issues and then she threatened our family and alleged our parents for finding unsuitable and incompetent groom which does not match her requirements. Our parents are senior citizens and even in mere pension he is getting, my dad called her from NCR 1 year back to home and provided her a rented place to live and my father is paying her rent, expenses for food along with the education fees of her 2 boys. Now she don't have job and everyday she is threatening and accusing our parents for her broken marriage & separation from her husband and all sorts of verbal rants which also includes use of abusing words, cuss words, spat on my father today called him a begger and cursing to threatening the name of parents and family by act of committing the suicide and defame the whole family. She is taking benefit of the innocence of our old age parents by threatening them daily. How can we proceed for legal action against her.?