@MeeshkaDiane
You can most likely forget about the alternate valuation issue since, per Section 2032, unless the alternate valuation decreases both the value of the gross estate and the estate's tax liability, it cannot be utilized.
Further, the 1/3 that you purchased from one of your brothers is controlled by Section 1.1015-4 (that 1/3 is considered to be part gift/part sale since you bought it below FMV from a close relative). As a result, your basis for that 1/3 would be the greater of the amount you paid your brother for the 1/3 or your brother's adjusted basis for the property at the time of the transfer.
See https://www.law.cornell.edu/cfr/text/26/1.1015-4
With respect to the remaining two-thirds:
The basis of the 1/3 you inherited would almost certainly be the fair market value on the date of death of the decedent.
With respect to the basis of the 1/3 you were gifted, you would typically need to know the donor's adjusted basis immediately prior to when the gift was made and also the FMV at the time the gift was made.
See https://www.irs.gov/faqs/capital-gains-losses-and-sale-of-home/property-basis-sale-of-home-etc/prope...
