Unauthorized interest claim

My father had to borrow a sum of RS,4,25,000 and RS 6,50,000 from two of my cousins respectively, and this amount was decided to be cleared off in subsequent 5 years time from May 2014(time of borrowing), however, at that point of time neither my father nor my cousin signed any document and the total borrowing took place under a verbal understanding of payment of interest to be not exceeding 1.5% per month. Now my father being an aged retired person who have become little unmindful as well,looking into the condition of my father's health my cousin got my father sign on place of blank stamp paper purchased in his name and shockingly he got the signature of my father when my elder brother and my mother was out of house. As and when the payment was made by cash by my father, my cousin has every point of time wrote it down in page of diary and surprisingly when he recovered the diary we observed that during 2014-2018 as on date,the total sum of rupees 4,50,000 to one cousin and 3 lac to other cousin,they have mentioned the recovery made was against interest. The interest value according to me is coming around 5% and 10% respectively per month. Under today's scenario we are totally in bankrupt condition, my mother and father are now pensioners and we are jobless being that we are going through acute financial crisis. We have in number of occasions ,shown and express our inability to carry out interest on huge amount which was never agreed upon. I am approaching to you to guide us for a solution ,so that, be then verbal understanding of 1.5% per amount between my father and cousins are adhered. Thank you.