How to stop and prevent defamatory letters

Hi, One and half year back I got married in India and within a week travelled back to USA along with my wife. Later got to know in USA that she is suffering from depression and incurable diseases and my inlaws hidden this fact from us and did marriage.My parents got to know about how we are trapped and cheated,my father expired of this shock.Later we filed a fraud case against them and sent my wife to her parents house.They retaliated with false 498a against entire family members.We are now on AB. Now my inlaws are continuously sending by post letters to my office both USA n Bangalore office stating I am a cheater, have harassed their daughter,taken jewellery, asking car,taken her passport, certificates etc.The false case of 498a is under investigation.Then what is the point of sending such letters to office attaching FIR copy of 498a.How is it going to help them? What measures can be taken to stop such defaming things to continue against me and my family.Please advise