Share of my married cousin

1. My grandfather bought property in 50s. Then he died in 1974. 2. His son (my father )got married in 1976. 3. My cousin ie my father's sister's daughter got married in 1984. Both of them my father's sister n her daughter were staying at my father's place as my aunty was widower. 4. Later my grandmother died in 2005. 4a. My single uncle died in 2013 without any heir. 5. My aunty who was staying at my father's ( grandfather's house) house also died last year. 6. Now my cousin ie my aunty's daughter saying that since her mother was staying there she has total right on that built up property. 7. My father being govt servant was away from his home with family since 1980. But he was regularly paying all taxes for that property. 9. He is ready give what is his niece's legal share, after proper discussion under law. But she is adamant on the point that she want built up property as her share. 10. Continuously sending notices to my father saying he has illegally occupied her property because right now after retirement my father has went back to stay at his house there. Property still in the name of my grandmother. 11. Plz suggest what is my cousin's that is aunt's daughter's share. 12. what is the legal way out for this mess.