Procedure to challenge Will being drafted illegally

Hi, I am a HINDU, I am from a HINDU UNDIVIDED family and last year my Grand father's elder brother expired(Sept 2017) and he has not left any WILL and he doesn't have any children. My father is having an elder and younger brother. Since my grand father had not left any WILL. The younger brother suggested to create a WILL to avoid the property sharing as per CLASS II heirs(Grand father had sister and brother and they have their family). He has taken the signature of my father well in advance before the WILL was drafted and even my father doesnt know the content of the WILL that he got drafted. Both the Elder and Younger Brothers are not in GOOD terms with my father. How can we get the copy of the WILL drafter by them and if the content of the WILL is not satisfactory to us(meaning the property is not shared equally among us) can we challenge it as being drafted illegally after the dead of the person. Please advise. Thanks