1.property is self acquired property of your grandfather which he bequeathed to your father under a Will
2. thereafter father made a Will of the same property to you and your mother
3. thus you and your mother got the property by testamentary succession i.e. under a Will
4. your mother was given a life estate and after her demise the property would become your absolute property
5. so i do not see how your sister can claim that it is an ancestral property and that she has a right in the property
6. also for a daughter to claim a share in ancestral property, her father needs to be alive on 9.9.2005 when the Hindu Succession Amendment act was passed which gave equal rights to daughters as that of sons in ancestral property
7. in your case your father died before 2005 i.e. in 1999, therefore your sister does not become a coparcenor and she cannot claim any share
8. your sister also cannot claim any share from your mother's share. Your mother was given only a life estate and no absolute ownership right. So how can your sister claim?
9. on one hand your sister claims that property is ancestral and father could not make a Will for the same and on the other hand she is claiming a share from your mother's share [your mother would have got a share in your father's property only when your father died intestate, in which event his property would devolve on his legal heirs being widow i.e.your mother, and his children. If that was so then the mother would have got an undivided share in her husband's property as a legal heir and upon her demise, her legal heirs, which includes your sister, could claim a share in her mother's share]
10 by claiming a share in the mother's share the sister has herself impressed that the property is a self acquired property and not ancestral property [since if the property were ancestral, then the mother would not get any share at all as wife of a coparcenor is not entitled to any coparcenory rights in ancestral property]
your questions:
1. your father's Will is valid as what he bequeathed was his self acquired property which he got under his father's Will
2. ancestral property is one which is inherited by a person from his father or father's father or father's father's father. In your case your father did not inherit the property from his father but got that property under a Will by a testamentary succession. Similarly you have not inherited the property from your father but got it under his Will by testamentary succession.
3. NO Simply because mother is excluded by your father's Will from inheriting his property. She was only given a life estate