When your father executed a Joint Development Agreement, he would incurred Long Term Capital Gains on the land value or the construction value, as the case may be. This value is the basis for determining the cost of acquisition for your mother. Her share of the land/construction value will be the cost of acquisition for your mother.
Thus in your case the valuation of the property in 2010 was Rs 23.70 Lakhs. If this was the consideration used for determining the LTCG of your father, then the cost of acquisition of the flat gifted to your mother would be 1/3rd of Rs 23.70, i.e., 7.90 Lakhs, assuming that all the 3 flats have equal valuation. Now the cost of construction of her apartment can be added to the cost of acquisition, whether incurred by her or your father. Assuming that the additional cost of construction was Rs. 2.10 Lakhs, then the cost of acquisition of the apartment sold by her would be Rs 10 Lakhs. This needs to be indexed to arrive at the indexed cost of acquisition. Assuming that the indexed cost of acquisition in 2017 is Rs 15 Lakhs, then the LTCG in the hands of your mother will be Rs. 10 Lakhs.
You can save taxes by investing LTCG amount in acquiring a residential house u/s 54 or investing in LTCG bonds u/s 54EC or 54EE, subject to fulfilment of conditions specified therein.
