I assume that your mother had a living trust - that was a revocable trust and became irrevocable after your mother's death. In this case assets left in the trust are considered as inherited.
The IRC 1022 is used to establish a basis for inherited assets acquired from a decedent dying after December 31, 2009 - http://www.law.cornell.edu/uscode/html/uscode26/usc_sec_26_00001022----000-.html
(a) In general
Except as otherwise provided in this section-
(1) property acquired from a decedent dying after December 31, 2009, shall be treated for purposes of this subtitle as transferred by gift, and
(2) the basis of the person acquiring property from such a decedent shall be the lesser of-
(A) the adjusted basis of the decedent, or
(B) the fair market value of the property at the date of the decedent's death.
However - the basis may be increased - up to the fair market value of the inherited property - but the aggregate basis increase under this subsection may not be more than $1,300,000.
So - in your situation - the basis would be a FMV at the time your mother died.
If you will sell shortly after that time - you may assume that the property value has not been changed - and the sale price may be used as a fair market value.
You also may find helpful IRS publication 559 - www.irs.gov/pub/irs-pdf/p559.pdf
Please let me know if you need any help.