Letter of administration or Succession certificate or Release deed or any two

My parents died intestate. We are only two sisters - both married. My parents own a flat in Mumbai where I was living with my parents. After my parents death,(father died in 2015 and mother died in 2021) my sister is ready to relinquish her full rights over the flat and make me sole owner. Please advise whether - 1. Release deed duly registered with stamp duty will serve the purpose(since it is inherited property - I presume that the registration and stamp duty is bare minimum i.e. around Rs.1000/- please confirm). Also sir inform whether release deed is to be signed and executed only by my sister or even by her husband and two adult children too. 2. Where along with release deed Letter of administration for flat or succession certificate is also required. Is both of these documents required or any one is required OR both do mean one and the same thing. 3. The flat in question is an inherited property. 4. Charges and time frame for obtaining Letter of Administration OR succession Certificate.