Money lending issues

My father had given an amount of 6 lakh rupees to my cousin bro in the kind of cosmetic items and he had promised my father that he would pay back the following amount which he mentioned on a blank paper with his signature and one witness which is his best friend and my cousin too, unfortunately he died all of a sudden in police custody because he has other cases on him in the middle east. My father went to his father to ask for the amount that his son had taken and showed him the paper that his son had written, he first said he would pay off the amount and the next following day he changed his mind and decided he will not given. Since then my father didn't take the trouble to ask the money back after almost a period of 8 - 9 years today his mother ( my father's sister) she says if you'll had an amount to take why didn't you tell us before, so they had a issue and she had complained to the police saying that the paper is fraud and it's fake and then the police bought us to an agreement to settle it by speaking and by swearing and then leave the rest to god, but later we got an information saying that they have filed a court case. I would humbly request you if you can enlighten me on this and what steps do we take and what is the best advice. Thank you.