My answers are as follows:
Daughter ( Bua ji ) got married and passed away in 2015. No biological children, has one step son - does he have right in my grand father s property ?
Ans: No, step son has no right.
Other son ( uncle ) passed away in 2000 - Has one Son : i understand he has right in my grand fathers property - what share ?
Ans: Yes, he has a right of inheritance.
Secon son ( Dad ) passed away in 2016. - Has one son ( myself ) and one married daughter. ( my sister ) - do we have equal share in our grand father s property ? Or it gets divided between myself ? My sister ? My cousin and my step cousin ( bua ji step son )
Ans: Yourself and your sister got half share each in the property fallen to the share of your father. And your uncles son get the entire share fallen to the share of his father. The law is as follows.
What does the grandparent's property law in India state? Does the grandson own the right to the property?
All property's owned by a Hindu person devolves onto his class one legal heir's.
Now to the specific scenario's in ur example (for sake of convenience I'm presuming ur ur grandfather has only one legal heir)
Senario1: The property is self acquired by your Grandfather, in such case upon his demise interstate (without a will) the property would devolve upon ur Father and not you. In case your farther passes away before your grandfather then it such case it would be devolve upon you, your mother and ur siblings equally.
Scenario 2: the property in question is self acquired by ur grandfather father ( ur great grand farther) - would devolve same as scenario 1.
Scenario 3: the property in question is self acquired by ur grandfather grand father ( ur great great grand farther) - would devolve same as scenario 1.
Scenario 4: the property in question is self acquired by ur grandfathers great grand father ( ur great great great grand farther) - then in such a case you would be entitled to the property by birth as it becomes ur ancestral property.
To give you more clarity on the concept of Ancestral Property's : any property which passes undivided down 4 generations of male lineage is called ancestral property. The right to such property acures at birth unlike other laws of inheritance where right arises upon the death of the the owner.
Hope this brings some clarity to your question and your sense of entitlements.
Section 8 in The Hindu Succession Act, 1956
8. General rules of succession in the case of males.—The property of a male Hindu dying intestate shall devolve according to the provisions of this Chapter—
(a) firstly, upon the heirs, being the relatives specified in class I of the Schedule;
(b) secondly, if there is no heir of class I, then upon the heirs, being the relatives specified in class II of the Schedule;
(c) thirdly, if there is no heir of any of the two classes, then upon the agnates of the deceased; and
(d) lastly, if there is no agnate, then upon the cognates of the deceased.