I understand that a commercial property was inherited by your family (i.e. 3 brothers and mother) on death of your father. Such property was converted into residential premises by paying necessary charges to Nagarpalika.
Since share of such property was not explicitly mentioned by your father, a view could be taken that the 4 individuals (i.e. mother and 3 brothers) own 25% share each in the said property. When such property is acquired by an individual by way of inheritance, such a transfer is not regarded as a taxable transfer under the Income-tax Act, 1961. Accordingly, no tax would be payable by any of the 4 individuals on acquisition of share in the ancestral property by way of inheritance.
As and when such property is sold in future, the cost in the hands of the previous owner (i.e. your father) or the fair market value of the property as on 1 April 2001 (where the property has been inherited prior to 1 April 2001) whichever is higher, would be deemed to be the cost of the acquisition of the said property for the purpose of calculating the capital gains.