Is willing a third of the estate to the children of the deceased brother, the children of the living brother, and the son of the living sister legitimate and valid?

Your question is ambiguous because you did not specify the exact relationship of the brothers and sisters (full siblings, paternal half-siblings, or maternal half-siblings). This affects the rulings on inheritance, so we cannot determine the division of the estate. As for the will, if it is for one-third of the estate, it is enforceable and distributed to the beneficiaries if they are not heirs. If the beneficiary is an heir, the will is invalid unless the other heirs approve it. Finally, matters of inheritance are serious and complex and must be settled through Sharia (definition) courts.