I am sorry for your loss.
The only type of appraisal the IRS has to accept is one conducted by a certified real estate appraiser where the valuation (fair market value) date is the date of death.
You should consult with a local tax professional because there is indicia here of a retained (implied) life estate, per Treas. Reg. §20.2036-1(c).
See https://www.law.cornell.edu/cfr/text/26/20.2036-1
In that event, the basis for you and your sister would be the full fair market value on the date your mother passed (with a long-term holding period since the interest was acquired from a decedent).
Again, consult with a local tax professional so the matter is handled properly.
See https://taxexperts.naea.org/listing/service/estates-gifts-trusts
