First you have to check the property is ancestral one or not? Property inherited upto 4 generations of male lineage (i.e., father, grand father, etc.) is called as ancestral property. Property inherited by a Hindu from his father, father's father or father's fathers' father, is ancestral property. Property inherited by him from other relations is his separate property. The essential feature of ancestral property is that if the person inheriting it has sons, grandsons or great-grandsons, they become joint owner's coparceners with him. They become entitled to it due to their birth. A person inheriting property from his three immediate paternal ancestors holds it, and must hold it, in coparcenary with his sons, sons’ sons and sons’ sons’ sons’ but as regards other relations he holds it and is entitled to hold it, as his absolute property.
So in your case on demise of Grandfather all his sons and daughters would have equal share in his property. So each son’s legal heirs have right over the property up to what their father can get. Male cousin's can sell their right over the property without the consent of other co owners (Sisters). If the person who occupied the property is not willing to partition then file a partition suit.