Is the father's will to allocate one-third of his estate to the poor and needy obligatory, given that the heirs of his deceased son are wealthy, and his daughters' share is small and they are poor?

If your father died after his son, then the children of this son have no right to inherit from their grandfather, because they are veiled by the paternal uncles. It is recommended for the heirs to grant them something from the estate. However, if your father died before his son, then the son's children have a share in the inheritance, which is the share of their father, and their comfortable financial situation does not prevent them from this.

As for the daughters' share, it is their legal share. They are not permitted to take from the money bequeathed to the poor and needy, because there is no bequest for an heir. However, they are allowed to give charity from their share to their poor husbands.

A bequest to the poor and needy is valid as long as it is within one-third of the estate. Its execution is obligatory, and it is forbidden to alter or change it.