بسم الله الرحمن الرحيم
When one year completes on the bank balance then you should pay its zakah as well as pay zakah of the gold available with you (though a complete separate year has not been passed on the gold).
(2) The amount which your father deposited in the account of your brothers, did he make them its owner? If he made them its owner then this amount and 5 tola of gold when combined together becomes equal to the silver nisab and thus they will have to pay zakah after completion of the year. But if at the time of completing the year the balance is short as it was spent and the nisab is incomplete then zakah shall not be wajib.
Allah knows Best!
Darul Uloom Deoband