Typically, when someone passes away, you should obtain an appraisal as of the date of death to obtain the fair market value that needs to be used for purposes of calculating the cost basis. It does not sound like this happened in this situation. Since your mom and brother held the property as tenants in common, only your mom's share of the property would be distributed to her heirs. Please see this link for more details on how this would be handled.
Since your mom owned 2/3rds of the property, that is the ratio that would be allocated to the stepped up basis. In this case, you would multiply 2/3 times the fair market value on the date of death to determine the overall stepped up cost basis for the heirs. The remaining tenant in common would retain their cost basis as the property was held as tenants in common, and as such, the remaining owner would not have a stepped up basis.
@focus123
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