Is what the grandfather and his wife did—writing all their property in the grandson's name—thereby dispossessing the father and the aunt—religiously permissible? And what must the grandson do to avoid what is unlawful?

If the grandfather and grandmother registered their properties in the name of a person without fulfilling the conditions for the validity of a gift (soundness of the donor and possession by the donee during the donor's lifetime), then the gift is not considered valid. The properties then become part of the inheritance and are to be divided according to Islamic law among the heirs. However, if it was a will bequeathing all the wealth to an heir, it is not valid except within the limit of one-third. Any amount exceeding one-third is considered detrimental to the heirs and must be returned to them. A gift made during a death-illness is considered a will.