propery dispute

My father having one brother and one sister.we having 20 acres land and one flat at borivali named on my grandfather name and land was named by my father,mother,uncle and aunty name. my father sister staying on our flat from last 35 years without and agreement now she demanding the share from land suggest me give me advice? she having 3 sons and 1 daughter all are working and even she get pension but my father and uncle both are farmer they do not having any income source.