Senior Citizen responsibility

My father was mentally ill since 16-18 years of age and was also kept in a mental asylum. His parents married him to a poor girl without telling details of his mental health as they believed money can get them anything. My mother left him in her forties and was staying with her daughters as it was difficult for her to bear any further physical abuse. His parents had huge self earned property which they divided by will into their other 3 sane sons with a verbal promise that they would look after my father. Now my father is 69 and my uncle is trying to dodge this responsibilty stating nothing is mentioned in will. He threatens me frequently saying as per supreme court he is your responsibility. I was married off when 18, have struggled on my own accord educated myself and am earning a living for myself, my children and husband( unemployed) managings loans and stuff. We have always contributed in cash and kind in times of need and have seen the money being misused and not used for him. My question is why children are responsible for him when he was married knowing his mental illness and parents did not leave anything for his maintenance. Inspite of an abusive childhood we managed to struggle and make our own life and now my uncle is threating often. It is not only a question of responsibility but already we are looking after our mother and as children we have only faced abuse, mockery and disturbed life due to my father sexually abusing my mother in front of our eyes. I feel my birth was not in my hands nor was it my mistake. It more of my mental trauma and dilemma