Legal heir of the property

My parents built a property in Delhi which was on the name of my mother. After the sudden untimely death on mother a decade ago my father continue living in the house. My mother never made any will and we continued the property ownership on my mother name till date. This year due to an accident I lost my father. He had also not made any will. We are three siblings and we recently signed an agreement (on legal stamp paper) stating that in the absence of will from my parents three of us will be the equal owner of the property and which can be sold only on agreement with three of us.We signed in the office of a lawyer but this document was not registered in the court. I am living abroad and my two siblings are local in Delhi. No one at this stage is living in the house with all the house keys are with my siblings. My questions are : - What do I need to ensure that they can not sell the property in any way without informing me or my consent - What is the validity of the mutual signed agreement on Rs 100 stamp paper. I appreciate your expert response.