As per Hindu Succession Act, your brothers widow and the childrens has all the legitimate rights and share in the ancestral property. since, the property belongs to your grandfather and hence the right devolves to all the legal heirs in the family. They can claim the share which your brother have in the property of your grnadfather atleast.
As far as the property self acquired by your father that cannot be claimed by them unless your father gives the share by way of gift deed, will or famliy settlement deed. Hope your query is answered.
Adv. Rohit m. Dalmia