Married grand daughter's claim on maternal grandfather's propertty

Before my grandfather died he gifted my aunt a small part of his self-acquired property which my aunt sold in 1992. Grandfather died in 1999 without a will and then grandmother in 2002. After that the mutation was done on my father and aunt's name. My aunt lives with her daughters, both married and have their own house. My parents and my brother live in the said property and I stay in a different city for my work. My father and I, being the eldest daughter, have spent fortunes to maintain our house and extend it. Now we are going through a financial crisis and wanted to build an appartment complex and sell the flats. Also I spent equal amount of money and care on my aunt as much as I do on my parents while my cousin's ignored her and used as a babysitter. Now my cousins are demanding through my aunt an equal share of this property while my father never said anything against her when she got the property as gift. We have no other property so after building the appartment we need to use 2 of the flats for ourself. If we have to give them half of it we won't be left with much money. Are we really bound to give them what they are demanding?