The sale of such residential property by your mother would be liable for LTCG Long Term Capital Gain.
To calculate the liable LTCG, you need the cost of acquisition COA. The cost of acquisition for property purchased before 1st April 2001 would be the FMV Fair Market Value as on 1st April 2001. This COA would then be indexed using the cost inflation index up to the year of sale. The net figure of the Sale Proceed and indexed cost of acquisition would be the LTCG taxable at 20%.
Regarding exemption, the available exemption sections are Sec. 54 and Sec 54EC.
Sec. 54 exemption would be available on the investment of the capital gain amount in another residential property within the specified period. This section doesn't have any condition of limit on the number of houses already owned, this condition is there in Sec. 54F.
Sec. 54EC exemption would be available on the investment of capital gain amount in specified NHAI REC Bonds within the period of six months from the sale.
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CA Hunny Badlani
Badlani & Associates