For starters the basis for property tax is not used on any income tax return unless all other alternatives to determining/establishing a cost basis have been exhausted without success.
You, as the last owner of the property prior to selling it just need to establish "your" cost basis. Your cost basis is the FMV of the property on the date of passing of the prior owner of the property. It does not matter if they passed years before you actually obtained ownership rights to it from the deceased owner's estate. It's the FMV of the property on their date of passing.
