Capital gain tax on gifted property, which was inherited by uncle from grand father

I'd like some help calculating capital gains tax. My sister inherited a house from our uncle, who in turn inherited it from our grandfather. The house is quite old (around 100 years) and we don't have a record of its original purchase price. My sister is now selling it for Rs 15,000,000. Can you advise on how to determine the capital gains in this scenario?