Try to keep questions, on the same topic to one post
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The cost basis of inherited property is the Fair Market Value (FMV) on the date of death. So, it doesn't matter what your mother paid for it. The estate executor may have done an appraisal, at that time. So. it may be in your records.
A little internet research can find you real estate historical prices graph for your area and that's close enough for government work. If you used a realtor, their estimate is probably good enough
I would use the property tax assessment, from that year. If you don't have a copy, you may be able to get that info from the tax assessor.
Can I deduct amount of taxes I paid while holding the inherited property until it was sold?
Real estate taxes (also called "property taxes") for your main home, vacation home, or land are an allowable itemized deduction if they're based on the assessed value of the property and the property is for your own personal use.