Property inheritance and Taxability

My grandfathers ancestral property was sold and the proceeds of which was used to purchase a property in Chennai. We are intending to sell this property . The property is in the name of my grandmother who is deceased. She has 2 daughters who are her Legal Heirs. The elder one is a PIO(US Citizen- Deceased) and the second daughter is an Indian Resident. The deceased sister is survived by her Husband who is a PIO (US Citizen) and her daughter (Born in US). A POA has been given favouring the sister residing in India for conducting the sale of the property on their behalf. Clarification is required on the following aspects: 1. Will this be considered a Ancestral Property, if so will all the Legal Heirs have rights. 2. Does sister’s husband have rights on the property, and further does the daughter have rights considering that she is born in USA. 3. If as per the above point there are no rights for them, then can my mother sell the property solely. 4. What will be the tax implication for US citizens for property sold in India. 5. Since the sisters family are not interested in the proceeds, can they give a relinquishing deed. 6. What is the procedure to get the relinquishing deed done in the US, and who is the competent authority to approve this.