Jewelry related

My brother had married 5 years back to my SIL who works in lonavala and have not lived with us more than a day or two. During corona and her pregnancy she was with her mother and My brother came back to us in mumbai and stayed with us post that for more than 2 years now returning back to his wife. Reason unknown. During thus last 2 years and after my father's death in corona he has mental tortured my mom and us two sisters alot against which numerous NC were done. Also he is unemployed and all the household and essential cost is taken care by me. His wife is not on good terms with us and since last 15 days he is returned back to her she is being threatening my mom for no reasons and also asking for the mangalsutra which was just given to her for display during her marriage. Another mangalsutra she wears is also given by my mother. Now she is threatening to go to legal and also accusing of theft and police complaint t. Since the mangalsutra is from my grandmother to my mother don't know want Canberra done.. please suggest