You might be able to use the FMV on the date of death (i.e, step-up basis) if you can show your mother retained a life estate (this would be an implied life estate).
See https://www.law.cornell.edu/cfr/text/26/20.2036-1
An implied life estate (which is the result you want) is dependent upon the circumstances of the transfer and the grantor of the deed.
See also https://patellawoffices.com/blog/planning-for-tax-minimization/navigating-inheritance-the-step-up-in...
