Can executor sell property?

My Grandfather passed away in 1982 and he has a flat in Kandivali West and my father and I have been paying all the maintenance and repairs for the last 40 yrs. Grandfather had 3 children (my father and two sisters) The Flat is a Condominium Society and still registered under my grandfather’s name. The two sisters have refused to pay any of the costs for the last 40 years. We have tried to settle with them, but they are waiting for my father to die. They tried to file to become executor but lost in court 4 years ago. My father wants me to be named executor because he is in poor health. Questions: 1. Can my father name me as executor for me grandfather’s estate? Or both Father and I become co-executor? 2. Since my father and I have been paying all bills – Is there an issue where the court would not make me the executor? 3. What is the cost to file in court and lawyer cost? 4. Once court names me executor, can I then transfer or sell the property without the two sisters approval? And distribute the cash from flat any way I choose? 5. Can they sue me in court if I sell the flat and keep the cash?