Are the sister's children entitled to demand their share of the valuable plot of land, after the deceased bequeathed it to one of his brother's sons, and the sister signed acknowledging receipt of her share of her deceased brother's inheritance, knowing that a bequest to an heir is impermissible?

A bequest to an heir is not valid unless approved by the other heirs. If the sister's children prove that their mother was deceived or signed the document while ignorant of her right to the bequeathed land, they have the right to claim the remainder of her share. Her signature in this case would not be considered an approval of the bequest. However, if they do not provide evidence for their claim, and their mother is someone who would not typically be ignorant of such rulings, then they have no right to claim. It is advisable to refer the matter to the Sharia (definition) court to settle such disputes.