Mother wants to sell flat after father's death can she do that

My mother wants to sell the flat and my father is no more ( 19 years back ,he died due to heart attack). But the flat was on his name that time and later it was changed to my mother's name . There was no wish made by him so that her name comes into picture .We are 3 siblings (elder brother ,elder sister and myself). To my knowledge , whenever such flat want's to be sold out then NOC is required from all of us. What is to be done in cases where anyone of us don't give NOC to mother ? Can flat be sold out ? And if yes then which process we need to follow ?