FD ACCOUNT INTEREST After death of holder.

my mother and grand mother had a joint fd account in co-operative bank and now my grand mother had been died before the maturing date now how much can we get the interest amount. say for my granny is senior citizen and she is 67 years and my mother is 44. my mother opened senior citizen fd account with a sum of 70000 for 1 year. but my granny died 4 months before maturity and the bank is deducting lumpsom sum of interest rate. and they said that they will close the fd account and open new account in name of my mother. is it necessary to close the accoint?