Is it permissible for grandchildren to take their grandmother's share of the inheritance from their father, who passed away before her, or must they give a share from the inheritance of their mother (the grandmother's daughter), who passed away later?

If the grandchildren have no one to prevent them from inheriting, then the grandmother's bequest to them is void because it is a bequest to an heir, according to the Prophet, peace and blessings be upon him: "There is no bequest to an heir." This prohibition is lifted if the heirs approve the bequest. The matter of estates should be referred to the Sharia (definition) courts for investigation and to safeguard rights.