1. This is not ancestral property to anyone.
This property was or maternal grandfather's own and absolute property actually it devolved on all his legal heirs after his death.
There is no automatic route for your grandmother to acquire the property after your grandfather's death.
Therefore this property will devolve on all the legal heirs of your deceased grandfather, i.e., your grandmother, and her five children.
Since one son is reported to have remained unmarried till his death, his share of property shall devolve on his mother who is the legal heir to succeed to the estates of her deceased son.
Therefore if the property was divided into six equal parts, your grandmother was entitled to 2/6th share in the property, she can dispose or transfer this share of property to anyone of her choice.
Similarly your maternal uncle is entitled to only 1/6th share in this property, which alone can be transferred to his son as a registered gift deed and not the entire property.
Your mother is entitled to 1/6th share in the property left behind by your deceased grandfather.
2. Your uncle can transfer his share in the property as gift to his son and not beyond that.
3. The grandchildren do not have any rights or shares in the property during the lifetime of their respective parents.
Your mother is entitled to 1/6th share in the property, she can fight for her rights by filing a partition suit