Documents to be provided by the heirs when the owner has died

My mother and my grandmother own a house in Delhi which is registered in their names. My grandmother passed away recently and we are in the process of disposing off the house to a prospective buyer who has to get a bank loan. My grandmother had executed a will in 2004 that is registered saying after her death, that her daughter (my mother) and her 3 grandchildren including myself would become the legal heirs of the house. Now we three grandchildren have no interest in the property and would like to relinquish and want the sale of the house to happen without any hassle. Would my mother need a surviving certificate? If so how and where does she get one from? What else would be needed to proceed? My mother is in Bangalore and has all her IDs of Bangalore. Please advise how to go about the sale without any legal problems both for us and the buyer with less of a hassle. I am a Christian.