If your parent used itemized deductions to deduct their state tax refund from their federal income tax in the year they paid the tax, then the refund would be added back to their federal income in the year they received the refund. That would make this "income in respect of a decedent" to you and your siblings. (But note, it would never have been taxable income on the PA state return no matter what.)
But the problem, of course, is that you don't know which tax return this applies to, whether your parent used standard or itemized deduction, and whether the refund would have been a taxable add-back, and not even the IRS keeps records more than 10 years. So I agree that you should just ignore this on your tax returns.
In other words, if it would have been taxable to your parent, it is taxable to you, but that is impossible to figure out at this point.
And, FWIW, Pennsylvania does not pay interest on unclaimed property.
