My husband had borrowed loan from my sisters but passed away recently without clearing the debts. My son just finished his studies and about to join his job, so what is Islamic fatwa to repay such huge amount. Please help.

Not sure where you are in the world but found these Islamic sites,

Debt & Religion: A Look at the Islamic View of Debt With Imam Asal - Get Out of Debt Guy

10.0. When is Zakat due? | Publications

Consulting with an Islamic cleric, or lawyer would be my suggestion.