Validity of will if benificiaries dies before Testator

My grandmother made a will bequeathing some properties to my father alone and not naming any of her other children and/or their heirs. My father passed away and my grandmother is still with us. Is the original will still valid? Do the legal heirs of my father have right to the property? Or do all the bequeathed properties can now be claimed by my grandmother's other sons and/or their heirs? Can I make a codicil to the existing will so that the heirs of my father receive the rights to the property after my grandmother passes? PS: My father did not have a will.