CGT calculation in case of jv with developer

Dear Sir,/Madam, My grandfather purchased a land in 1952 & build a two stored house in that property. My grand father expired in 1964 & my father became the legal owner. My father passed away in 1983 leaving behind my mother ,myself & my elder sister. My mother passed away in 2010.Now myself & my sister has signed an agreement for JV development with a builder in 2021, in which the developer will demolish the existing building & construct a four storied building comprising 4 flats i.e. One flat in each floor. Out of these four flats two flats will be sold by the Developer & one flat will be given to me & one flat to my sister. I will be staying in the said flat but my sister will sell off her flat during construction period before possession. As both myself & my sister will be joint owners for both the flats allotted to us by the Developer. 1. Do I have to pay capital gain tax for for retaining my flat for my own stay ? 2. Do I have to pay GST to Developer before taking possession of my flat? 3.Do my sister have to pay GST as well as Capital gain tax during selling of her flat? 4.If so how CGT will be calculated for my sister & myself ?