Challenging a Registered Will

My grandfather expired in 2014 at an advanced 90+ age (we do not have clear idea about his exact birth date), since then my grandmother has expired too. They had 8 children, 5 sons (2 deceased) and 3 daughters. My own father, his son, expired in 2000. Soon after he (my father) died, other 4 remaining sons conspired and created (together and separately) wills that disinherited my father or his legal heirs (me, my brother and mother) from the entire property. They are claiming in the will, through my aged and ailing grandfather, that my father's own residential property, registered under his name and later sold by us in financial difficulty after his demise in 2002, was actually paid for (construction and acquisition) by my grandfather. Therefore, my father and his legal heirs are not to be given any share anymore. The act of selling the house is what triggered the changes in the will in 2003 and 2013. The remaining four sons have not been on good terms within themselves and have at different times (2002, 2003 and 2013) since my father's demise have created multiple wills favoring themselves all signed by my grandfather. The last two wills categorically exclude us from any share in the property. One more son has passed since then (2010) and his widow has been allotted a hefty share in all the properties, singling out my mother as the only widowed daughter-in-law who was not awarded anything despite my grandfather telling her that he has taken care of us. Two Wills in our possession from 2000 (made two months before my father's demise) and 2002 gives my mother some share in two of the listed 8 properties. All disputed properties are self acquired by grandfather. Some oddities in the Wills: 1. 2000 Will (when my father was alive but hospitalized) states that all five sons are capable and caring and does not mention my father's individual property but 2002 Will (created after my father's demise) states that 'he (my father) never lived with the joint family since his marriage and job (which is untrue we lived there till his demise) and had an intention to claim his share early on, so I (i.e my grandfather) helped him with monetarily with acquisition of land and later the loan'. 2. The 2003 Will still repeated the above language of 2002 will except that my mother has been ousted from all the properties citing the same reason. If helping my father with his house was not a reason enough to oust my mother from 2002 Will then why in 2003 my grandfather decides that he does not want to provide her a share based on the same. 3. The 2013 Will cites 2002 as the last Will, but the eldest son is claiming the 2003 Will is the latest, we do not possess this one but know that it also disinherits us. 4. The 2013 will again repeats that we never lived with the family but we have multiple ID proofs, Passports, Ration card etc that have the family home as permanent address. 5. 2013 Will states that my grandfather helped my father with the loan completely but my father had the loan's EMI cut regularly from his pay as a PNB employee. My questions are- 1. What is our legal standing on challenging the will/s ? 2. Which one should be challenged? 3. What proof should I offer (I do have conveyance deed proving my father's house was under his name and not my grandfather's property and I can acquire loan receipts from PNB)? 4.Can I challenge a registered will that has all my grandfather's self acquired property and if yes, then on what grounds? 5.Can a Will be voided because it wrongfully states that the land for our house was bought and construction paid for by my grandfather? 6. Will the court be suspicious of our disinheritance particularly after my father's demise?