Grandparents made a Joint Will. There after grandma made another will.

my grandparents made a joint will in goa stating that each of their grandchildren will get flats in all of their properties. my grandfather passed away in 1999 and my dads sister made my grandma make another will in 2000.there was no transfer probate done for the will made in 2000. . my grandma passed away in 2014.my aunt has made her put on the will that she will be the landlady after her death to the properties.my question is can the joint will be revoked can we contest the latest will in court can she do anything without my father n other siblings signatures most properties r joint name