As far as I understand RTI, the Bank can refuse providing this information not because this is your information, but because the following information cannot be given: "information which relates to personal information the disclosure of which has no relationship to any public activity or interest". But you may try your luck - send them the RTI application alongwith DD/PO by post. Then they will be obliged to refuse in writing.
The RBI rule that the Bank is talking about is perhaps the Banking Companies (Period of Preservation of Records) Rules, 1985. Your statement does not come under this - this is about retention of chqs, account opening forms, vouchers etc.
I think yours is a customer service issue. The statement can be easily retrieved from the computer and given to you. The Bank probably does not care because you/your father is not a customer anymore. Write to the branch manager, then to the Regional office, then to the Central office, with a gentle threat to take matters to consumer forum/courts.
You can also try a different approach: have Mr X (your friend maybe) who talks well meet the branch manager, and let Mr X tell the BM that he intends to open a current a/c alongwith family savings a/cs, as well substantial investments in Life Insurance (all banks are looking for business in LI). If the BM's tongue is waggling, let Mr X mention (on the 2nd/3rd meeting) that his friend (You) had a customer service problem with this branch and since he was not satisfied, Mr X is thinking of having this relationship with some other bank. I think if the BM is convinced about Mr X, the statement will be provided in 5 mins. If you & MR X plan this and execute the plan well, I think this will work.