Need advice on stamp duty

We are a family of 2 brothers and three sisters. My father had expired long back. Last year my brother has died. Our family has mutually decided to do relinquishment deed in favour of me and my nephew having equal share. We are planning to construct 3 floors on our land. First two floors will have 1 flat per floor. Third floor will have 2 flats of equal area. I want to add co owner as my wife in 1st floor flat & my mother as co owner in top floor flat of my share. Please advice the procedure involved & how much stamp duty would be applicable. Thanks