Is zakat due on money collected from a gift by a deceased grandfather, and how is it calculated if due, in terms of the nisab (based on the current or previous price of silver), the hawl (one year), the number of years for which it must be paid, and does this include the years after the grandfather's death, and does its payment continue, and is it also obligatory for the brothers?

Zakat (definition) is obligatory on the money your grandfather gifted you if it reaches the nisab (definition) (minimum threshold) and a hawl (one lunar year) passes over it. The nisab for silver is 595 grams, and for gold, it is 85 grams. The nisab is calculated by combining it with any other money, gold, or silver you possess. The amount of zakat is a quarter of a tenth (2.5%), and the value is calculated at the time of disbursement. Zakat does not lapse with the passage of time; it must be paid for past years in which the money reached the nisab and a hawl passed over it. The same applies to your siblings.