You both as children of father have absolute right to share in the property that has devolved on your father under the decree of partition in 2002. You can claim and have your share in the property. Issue lawyer’s notice  to family members seeking partition  and separate possession of property. After that file a suit in district Court for partition and separate possession. You will get your share.

You don’t have any rights on property by birth 

on father demise intestate you and your sibling would inherit his property 

Both of you have a right over the property. 

The share of property inherited by your father through a partition deed shall become his own and absolute property.

Since this is not an ancestral property you nor your siblings can claim coparcenary rights in the property 

Therefore neither you nor your siblings have any right for a share in the property at least not during your father's lifetime.

Yes the grandchildren have a share in the property of the grandparents.

Dear Sir/Ma'am,

As per the Hindu Succession Act, 1956, grandchildren do not have any birth rights in the property of their grandfather. 

A grandfather is free to transfer the property to whosoever he desires during the course of his life.

However, if he dies without leaving a will, only his immediate legal heirs can claim the right to have a share in the property. Immediate legal heirs include, wife, sons and daughters.  

As your grandfather got his property through a partition with his siblings in 1956, the ancestral nature of the property was lost at that time forever. His share became exclusive and absolute from then on. After his intestate death in 1981, your father and his siblings got their respective shares through a court order. As such, neither you nor your sister has any right over your grandfather's property. You have a claim only on your father's share. Your father may either opt for a settlement or write a Will in favour of you and your sister, if he so wishes.