Though this was your grandfather's grandmother's property, since it devolved on him and he was in possession and enjoyment of the same till his death, the property after his intestate death devolved equally on his own children consisting your father and his two sisters.
Therefore as per law your father shall be entitled to only 1/3rd share in it.
There is no question of ancestral property since the property automatically devolves on the three.
An amicable settlement or partition shall be the best option available before everyone or your father may approach court seeking partition and separate possession of his share in the property.