Who is the owner of the house if the grandparents died intestate

My Grandparents had bought a house 35 years ago. Both of them died intestate. Both their names were cancelled on the Share certificate and my mom's name was added. There is no will, no POA. We have been living in the house for more than 30 years. Electricity bill and telephone bill and maintenance bill is taken care by mother . The Electricity bill is in the name of my Grandfather whereas the telephone is in the name of my mother. My question is if we want to sell the flat, what documents need to be in order. Also is my mother the owner of the house?