Mere NOC is not sufficient 

Registered deed of relinquishment ought to be executed by daughter to relinquish her share in property 

NOC is not valid, she have to give POA to any of them to execute sale deed on her behalf. She will include as seller.

Yes, daughter or her legal heirs can challenge that NOC its better to present her at the time of sale deed.

See since the property was in name of the father and he has signed sell deed and daughter has given NOC then in future there won't be any issue in the future.

1. No. the children of daughter have no legal jurisdiction to challenge the said NOC.  However, legally it would have been more prudent to take a Power of Attorney (POA) with proper relevant /strategic clauses.

2. Even today you can take a "irrevocable indemnity bond" with strategic clauses, from daughter, to indemnify all monetary & legal liabilities related to the ancestral property.

- As per law, now a daughter whether married or unmarried is having equal share in the Ancestral property i.e.  an equal share in such a property accrues by birth itself. Before 2005, only sons had a share in such property. 

- Hence, NOC is not enough , and further it is also not valid document in the eye of law, for leaving  a share in the property.

- Further, she or her children can claim her respective share in future. 

- You should take a Release / Relinquishment deed from her , for leaving any right over the ancestral property. 

If it's an ancestral property her children can also challenge the transfer

If property is ancestral then her children can file claim on future so you should also take NOC from her children. 

In what way it is said to be ancestral property.

If it was your grandfather's property and a share was  allotted to your father by way of  partition or settlement or any family arrangement among his siblings, then it becomes your father's own and absolute property.

Thereby he need not obtain NOC or relinquishment deeds from his children to sell the proeprty to the prospective buyer.

He need not even take the consent or permission from anyone including his children to transfer the property to a third person by way of registered sale deed or any other registered deed.

This implies that he need not obtain NOC from daughter for selling his share of property to a prospective buyer.

The NOC obtained in a NJ stamp paper is of no use especially when the person who has a right in the property wants to relinquish his/her rights in favor of other shareholders, it should be done by executing a registered release deed relinquishing his/her rights in favor of others, hence the proposed NOC has no legal validity either now or in future.

Daughter can challenge NOC, daughter should execute relinquishment deed it should be duly stamped and register. 

Dear sir,

NOC on such stamp paper is not proper . She should execute relinquishment deed which must be registered then only father and two sons can sell the property legally otherwise issues will arise at a later point of time .