Dispute on loan

Dear sir, myself kumar baibhav from ghatkopar, mumbai. My mom had taken a loan of rs.50,000 from a lady 3 years ago named renu. she wasnt into this money business of giving loans but just wanted to make some cash from monthly interest of rs5000 so she dosent have a proper licence for this. My mom had agreed that she had received this amount of 50,000 through a stamp paper but nothing was mentioned about the interest as they said that interests are not mentioned in legal papers(i guess they fooled us). There was one more lady who was the witness that time,actually she was the one who introduced my mom to renu.Her name is Kamal. Kamal used to take intrests from my mom every month and would give it to renu. and this went for almost two years. last year renu said that she wanted her money back (50,000) but my mom wasnt capable to pay that time so renu agreed and said fine, keep paying interests then. But actually what had happened was, Kamal decided to pay 50,000 to renu and she herself started keeping all the intrests. It was now like the amount was switched from renu to kamal. and no legal paper was made on this. my mom was also ok with it when she came to know about this.Today we are in a financial crisis and my mom has stopped paying interests to kamal from around 6 months and has told her that she would pay the principle amount not at once but in terms of small amounts of 10,000 every month. but know the ugly thing starts, Kamal is saying that she wants her interests first for the 6 months and then she would think about the principle amount. she's threatening my mom and mentally harassing her. we have paid huge price for almost two years in terms of interests. even kamal knows that interests on this amount are illegal but she's threatening us boldly because of my father. he dosent know anything about this. my father is a heart patient and i fear he wont be able to bear this.she says she would molest our family and all other sorts of abusement.. plz help.. is there any kind of legal action can we take against her and is it first of all legal for a civilian to charge interests on money without license and also to be known that there's nothing about the interest on legal paper that my mom had signed and that paper relates to renu and not kamal... plz help sir.. waiting for a reply......