How to get legal heir certificate when parents did not marry legally

Dear All,My Mother is holding a Staff Loan Account with State Bank of Hyderabad, Vanasthalipuram branch. She has passed away in the year 2009, leaving the housing loan uncleared. However I have cleared the entire debt and now the Bank's Law department is asking for Legal Heir Certificates to handover the Title deeds.The main issue here is my mother is not married to my father legally as per the hindu law and there is no marriage certificate to prove it.After my father died, she started living with another guy(not married legally as he is having a wife by that time from whom he didn't obtain divorce from the court of Law). As a result of their living together, my sister was born. After all these, my sister and myself are good in terms with each other.We have been receiving Pension under family pension scheme recognizing both of us.