Right of grand children in grand father's property,in absence wil

My father died in 1988 with out writing the will.we were three brothers and no sister.my youngest brother expired in year 2005.he left one daughter and one widow.i being the eldest son had built two floors (ground floor and first floor)and my middle brother built the third floor.my youngest brother never spent single paisa on building.Now the widow of my youngest brother and her daughter want the ground floor registered in their name.this house is not earth quake resistant and is built on brick wall with lime and fly ash mortar.i want this house shall be built on REINFORCED concrete frame conforming to earth quake resistant code. i asked the widow of my daughter and her daughter to pay for the construction because you have not spent any money on its construction.in year 2010 all members of family i.e. my brother,widow and daughter of my late brother relinquished their rights in my favour and are registered in sub registrar,s court.now they are compelling me to write the ground floor in their favour,which they have never built and occupying against my will.now they have served me a legal notice.widow of my brother retired as class ii officer from income tax department and her daughter is chartered accountant getting approx Rs 50000/ pm. where as i am retired since 1998 and living on my savings.kindly explain that what are the legal rights,when widow and daughter of my diseased youngest brother already relinquished their rights in my favour. i am a hindu.my youngest brother died after my father's death.