Inheritance of PPF account of deceased father

1. My mother died on 15Jan16 and my father died on 15Nov16. 2. My father had a PPF account with SBI. Current balance is ~18Lakhs 3. When we had approached the bank b/w 15Jan16 & 15Nov16 -- to check the nominees -- we were told that I was the only nominee. Hence we left it as is as per my father's wish that I should be the nominee. 4. When I approached bank in Apr17 to claim the PPF account proceeds, I was told by SBI that in this PPF my mother and I were nominees. I asked them that since the other nominee has also passed away, the proceeds should be given to me. Bank insists that they would treat this as a 'no-nominee-case' and hence the proceeds will be equally divided between me and my sister subject to submission of indementies, surities (from 2 persons with account in SBI with balance in excess of ~18lakhs etc). 5. I do not know anyone who has an SBI account with 18 lakhs balance and who can give a surety for 18 lakhs (that too two persons). My sister does not want this money. I think the bank's ask for surety etc is absurd as one nominee has died and other is alive so money should be given to the nominee who is living. 6. Can one of the experts guide me how to convince bank to give me proceeds without the need for sureties etc?