Sale of an inherited home

My mother passed away in 2015, leaving her house to my sister and me. For various reasons, we've let the house sit vacant since then. We've just sold the house and now we need to figure out how to determine the Fair Market Value as of 2015.

We had a comp run on the house by a real estate agent a few months ago and ended up selling the house for a little less than half of the comp number. I'm seeing where the property tax assessment isn't a good guide as to the value of the house in 2015, so I'm stumped as to how to determine the FMV as of 9 years ago.

Any ideas?

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