Old flat in kolkata, name transfer, legal heirs involved.

My father (Hindu) purchased a flat from a relative back in 1991 about 26 years ago on the basis of a agreement of sale but it was not executed so the flat is still in the ex-owner's name in the registrar's office but we are staying in this flat since then.. Now both my father and the ex-owner has passed away and I approached the legal heirs of the ex-owner somehow through social media and they have agreed to transfer the property name to us in a positive way without asking for anything as we have been already staying here since 26 years and they know my father paid their father full payment.. I want to get the transfer done from the legal heirs(2 Sons and a daughter(married)) to my mothers name ASAP before they change their mind or demand seeing the current situation.. 2 Sons are staying in hyderabad and do not want to come to Kolkata but they are ready to sign the document if couriered or somone told me to get power of attorney of both of them to their sister's name here in kolkata and the sister can alone transfer the property to us here in kolkata. The sons never knew that this property was still in their father's name and have never shown in their balance sheet so they do not want any capital gain tax burden as such either so they want a win-win situation for both parties.. a) I am confused whether to go for a gift deed or Transfer the property as per unregistered sale deed by paying requisite stamp duty & penalty. b) Should i get the gift deed template prepared first or get the POA first from hyderabad... c) Will there by any capital gain involved since there is no sale now..either gift or transfer through old sale deed.. d) What all documents required...?? This is first time for me and looking for suggestions..