Assuming this is property tax issue:
Typically, tax, penalty, and interest owed is tracked and reported by tax year. It is also often paid by tax year. Call the taxing administrator (whoever calculates, sends bills, and collects taxes), and ask for a current statement of tax, interest, and penalties due for all tax years. If you don't understand what is provided, then ask them.
If it's your grandmother who owes back taxes, then that's a very stupid way to lose your house for pennies on the dollar. She needs to get her act together.