In ordinary course, the survivor becomes the single holder when the joint holder expires. Thus in your case, you should have become the sole depositor upon expiry of your grand mother. If the bank converted the joint account into single account as is required to be done, the PAN would have been yours when tax is deducted at source on interest credit. So pl. check if the PAN is yours in 26AS in the TDS certificate.
The interest income becomes your income after the death of your mother. If the interest is payable on maturity, you can offer them as income in the year of maturity and claim TDS in respect of such income, though deducted in the earlier years.