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.
