But some of family members are saying that the property will be divided in to 15 parts (5 parts - Grand Father, 5 parts - Older Father and 5 parts - My Father) due to my Grand Father inherited from ancestors and he died before 1985. In those 5 parts of my Grand Father, 4 will come to my Father & 1 will go to my older Father. So finally my older Father will get 6 shares and my father will get 9 shares. Like that they are saying.
What you heard from others which has been stated above are incorrect.
First of all it is not ancestral property becasue your grandfather inherited the same from his father, hence it lost the character of ancestral property when your grandfather inherited the same from his father.
Subsequently, upon the intestate death of your grandfather, .the property devolves equally upon his own legal heirs consisting his wife and children.
Therefore your father is legally entitled to 1/5th share in the property.
If his sisters are willing to give their share to your father, then they may execute a registered release deed relinquishing their rights in the property in your father's favor.
Your grandmother's will shal come into force only after her lifetime, hence her share of property is not confirmed to your father yet, your grandmother can change her will anytime during her lifetime.