1) how did your grand mother inherit property on your grand father demise?
2)if your grand mother has left a will and bequeathed you the property you would inherit her property under the will .
3)if your grand mother died intestate ie without a will you would inherit your father share along with your father second wife and your step sister .
4) you have not mentioned whether your father had any other brother and sister . in the event your father had any siblings they would also have equal share in your grand mother estate