You need to first get it's valuation report from a government approved valuer for 01.04.2001 and then index it using cost inflation index provided by government to give inflation effect and then find the capital gain.
You can save Capital gain by either investing in bonds u/s 54EC or buying a house property u/s 54F.
No your sister won't have to pay any tax as it's a gift from her mother.
But if your mother gifts the amount to your sister she might have to pay tax.
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