Fraud(420) by Daughter of Kalyug

My 68 year old mother had made 2 fixed deposits of total 14 lakhs in a Bank in Mumbai in September 2009 from her own savings from her single savings account in her Single Name.In December 2010 my sister trapped my mother and got her signature and got her name joined as a second account holder in the said 2 fixed deposits.My mother can not read and understand English so she signed on the basis of trust.My sister has also kept original Fixed Deposit Receipts with her since then.In Feb 2013 when my mother asked her for original fixed deposit receipts,sister flatly refused to give them back and asked for 50 percent share ie. 7 lakhs in cash.My sister or husband have no source of income and they did not contribute a single rupee in this amount.Despite many relatives convincing my sister, she has been acting smart and arrogant and says "Do what you can".The Bank has now put both Fixed Deposits on hold due to dispute.We submitted an application for FIR to Police Station in Mumbai under IPC section 420(Cheating) & 511(Attempt to cheat).But police took no action saying it is a civil matter.We want Police to take action against my sister as it is not just a question of money but also self respect and justice.Is this what we get by giving equal right to daughters?and Police says that they r doing everything to ensure woman's safety in TV interviews while a 68 year old woman's complaint is not even investigated and she is asked to go to court.