Dear Sir,
With respect to your quarry please find below answer :
1. Cost of Acquisition shall be cost of purchase by your mother's sister. if same is not available you can find out through prevailing jantri rates at the time of purchase for calculation of index cost.
2. & 3 No the option of investing in third residential property shall not be available to her. She can only save capital gain by way of investing amount in NHAI & REC Bonds.
For further details please free to contact me.
Thanks & Regards
CA Vishrut Shah
(+91-98214798534)