1. The property is actually not ancestral because once it was partitioned among the brothers, it lost it character of ancestral even it is presumed to be one .
In my opinion, this property might have belonged to yor grandfather which was partitioned among his sons.
If yes, then it cannot be termed as ancestral property.
Therefore the property bequeathed by yor father through a Will is absolutely valid, provided he had written it while enjoying a sound mind of health and his signature duly attested by two witnesses.
Any attempt to force your father to cancel the will especially while he is affected by dementia shall be invalid and illegal in law.
Also nobody has a right in it nor can claim a share in it as a right.
If your unmarried sister is not cooperative you canot force her to cooperate for dividing the property which belongs absolutely to your father at least during his lifetime.
2. If this property has not been divided by yor father then neither your sisters nor your other or you can claim any share in the property as a right during your father's lifetime.
3. Basically your notion about the ancestral property is incorrect. The property was inherited by yor father as his share hence he is the absolute owner of the same, nobody can claim any share as a right on any basis during his lifetime.
The will what he has written will prevail after his lifetime if the same has not been revoked during his lifetime.