In such cases, the first step would be to complain to your Bank via the grievance redressal process of the bank. For this particular bank, the procedure for complaints is given here: http://www.hdbfs.com/Grievance_Redressal_Process.htm
. If the bank does not provide a satisfactory solution, the next option would be to file a complaint with the Banking Ombudsman, a person appointed by the Reserve Bank of India to redress customer complaints related to deficiencies in banking services. The contact details for the fifteen Banking Ombudsmen in India presently can be accessed here: https://www.rbi.org.in/Scripts/bs_viewcontent.aspx?Id=164
The Banking Ombudsman is empowered to consider disputes relating to non- adherence to the bank’s fair practices code as well as the Code of Bank’s Commitment to Customers, which this bank, as a member of the Banking Codes and Standards Board of India is required to adhere to. The Bank’s Fair Procedure Code provides that the Bank must give notice to the borrower of any change in the terms and conditions, in a language understood by him.
Furthermore, the Code of Bank’s Commitment to Customers contains the commitment that the bank is to deal with the customer with transparency and integrity
and that clear information is to be provided about the products and services, along with the terms and conditions that apply to them. 
Misrepresenting that your mother was being asked to sign for witness purposes, adding her name to the loan structure, adding a fake signature to the sanction letter and finally evading your request to look at the insurance proposal form are all in violation of the undertakings of the Bank in the above mentioned Codes, and thus fit cases for the Banking Ombudsman. It is important to note, however, that the complaint must be filed within a year of the cause of action having arisen. Therefore you must file the complaint within a year from the date discovery of the fact that your mother’s signature had been forged on the sanction letter. The complaint may be filed online.
However, before you can file a complaint with the Banking Ombudsman, it is essential that you attempt to resolve the complaint with the Bank itself.
Besides the abovementioned facts, it is important to note that forgery is a criminal offence under the Indian Penal Code.
Therefore, it is always open to you, in addition to the other remedies to file an FIR against whoever was responsible for falsely putting your mother’s signature on the sanction letter.
Available at < http://www.hdbfs.com/Fair_Practice_Code.htm) > Last seen on 17-02-2016
Refer to Section 2.1.1 of the Code. Available at < http://www.hdfcbank.com/assets/pdf/RBI_English.pdf) > Last seen on 17-02-2016
Section 2.1.2 of the Fair Procedure Code.
Available at < https://www.rbi.org.in/commonman/English/scripts/AgainstBankFAQs.aspx.> Last seen on 17-02-2016
Section 465 of the Indian Penal Code, 1860