Will drafted in 1996, the testator and the two witnesses are not alive

My gradndfathers elder brother having no legal issues, drafted a will in 1996 on a normal paper, in favour of my mother. But now, neither my grandfather nor his brother or either of the witnesses are alive. Is it possible that the will could be challenged by my aunt who has no share in the property but her sons have been granted 20% share in the will. THe will has not been registered or probated. Will my mother get her 80% share in toto.