Mother want to transfer her share what are the legal procedure

My mother, sister and me are co-owners of a flat purchased in Mumbai and registered in April, 2015. My mother would like to transfer her share in my sister's and my name. 1. Should she do this through relinquishment deed or gift deed ? 2. Will we still have to pay registration charges if we do a relinquishment deed? 3. What documents are to be attached with the deed ?