suspicion over latest registered will

My grand father made two wills in his life. He died in 2008. Both wills are registered and the latest one is in my father"s favor. 1. Grandfather had two children: One son (my father) One daughter (died in 1986 - murdered by her husband) 2. The first will was favouring the sons of my deceased aunt. 3. The equations changed over the years and in his last will he made my father the major beneficiary. 4. The sons of my aunt have challenged the will on below points: a)They have their names on ration card with my grand father. where as my father was staying in seperate house in the same village b) They have made mental incapability of the teastator a ground for their appeal. But have not provided and doctor report, doctor or witness to appear in the court c) My grand father made a register POA in my mother's name on the same day he ,ade his registered will. My father was not there at the time when both the documents were made. POA was never excersised. d) Grandfather was not able to move and sign as he was having bed sores and effected from parkinsin (shivering hands) e) The testator gave application to the registrar along with his medical report to come to his home and register the will. f) Will was registered at home and testator mentioned all previous wills revoked without giving exact details of previous will. The entire case is challenged by plantiffs (mu aunt"s sons) on the basis of suspicion. Can you please guide me on the merits and demerits for our side. Thanks,