Gift Deed and Stamp Duty on My share of a Flat

Hi, I had purchased a flat in Mumbai-Maharashtra in 2011 with my own money and had put the name of my wife and mother as joint owners. (The flat was bought from my money only and neither was there any monetary contribution from my wife or mother. Each one of us hold 1/3rd share in the flat). Now as my mother is old I want to execute a gift deed whereby my mother will be transferring her share (1/3rd share) in the flat in my name. I am already my mothers nominee in the Society's records. I have 2 siblings who have legally / financially nothing to do with this flat as its my own personal flat bought by me and not inherited by me. I have 3 basic queries: 1. Will stamp duty have to be paid by me only on the 1/3rd share that is going to be gifted to me by my mother? Or on the whole market value of the flat? 2. Will indexation rule apply while calculating the stamp duty? What is the stamp duty payable in Mumbai Maharashtra 3. What happens if my mother passes away? Can my siblings stake a claim to her 1/3rd share them being their legal heirs along with me?