1. By your narration it is understood that the property is still in your grand father's name.
2. Since your grand father had written WILL, duly attested by two witnesses, in favour of your father, the question of taking NOC from your chacha's daughter would not have arisen, if the WILL were to be probated.
3. As it is, your chacha's daughter does not have any claim on the property since her grand father's property was not willed in favour of her father. However if she challenges the authenticity of the WILL in a court, then it depends on the ruling of the court. If the court set asides the WILL, then the property would be shared in 2 proportions, i.e., your mother & you 3sisters will get 1 share and your chacha's daughter will get another share, viz., 50% share to your family(comprising your mother & you 3 sisters) and the other 50% to your chacha's daughter. Please remember that this property is not ancestral and the property would devolve to the beneficiaries as detailed in the WILL.
4. As per the contents of the WILL, your mother & you 3 sisters will have 1/4th share in the property.
5. Since you & your sisters have released your individual rights in favour of your mother through relinquishment deed the mutation of the property would be in your mother's name in municipal records.
6. It is for you to consider or not, whether to go for settlement deed with your chacha's daughter, if you feel that the WILL, if challenged will not stand the test of time.