Does "the obligatory bequest" mean that the sons and daughters of the deceased sister inherit from their deceased grandfather?

If your father did not leave any heir other than you, then you take the entire inheritance, and there is nothing for your deceased sister's children, because they are not among the heirs, but rather among the relatives by blood (dhawu al-arham). The obligatory bequest (al-wasiyyah al-wajiba) refers to the law applied in some countries, which mandates for the grandchild a share from his grandfather's inheritance similar to his father's share had he been alive. Matters of inheritance are very serious and extremely complex, so the inheritance should not be divided without consulting the Sharia courts.