Mental harrasment by my sisters husband to my parent

I will explain the case point wise:- 1. there is some diffrence in opinion with my father and my sisters husband regarding some family matter. 2. based on that issue my sisters husband is mentally harrasing my father with sending abusive letters and phone calls. 3. initially we thought he will stop this by himself after some time but its now getting more & more uglier as he threatening to divorce my sister. Its now 7 years and he is not stopping all this. 4. I want to mention in this case that my sister always takes his husband part. because of that we are unable to take any step forward as every time we think he will stop these activities. 5. Due to this continuous mental harrasment my parent heath is getting detoriating. We tried to resolve the issue with talking to their family but no help. again after some time he starts all this once again. 6. Now we want to take some legal action in this regard, please suggest how to proceed as we are so confused. we have all his abusive letters which he has sent us with fake address. - Kindly suggest some option as we are fedup with his activites.