Presuming you are a hindu family and you are two sibling. Once your father died interstate the ancestral property devolved on you and your sister and your mother equally if she was alive at that point of time. in such case the son of your sister is having the right over the property.
if the daughter has died before the father, her legal heirs cannot claim a share in the property.
Contact: firstname.lastname@example.org in case of any clarification.