My aunts approval necessary for getting a will from my grand parents

My great grand father left 0.5 acre to my grand father. My grand father had self accrued property of 5 acres. My father has myself and my younger brother and left us. My aunt didn't have child adopted a child. Grandfather (age 88) due to his age has identification issues sometimes only (We suspect my aunt has medical records related to this). Now my grand father wants to transfer property to me and my brother, not to my aunt since she already got enough gifts. But my aunt is not agreeing to sign. 1. Without my aunt's knowledge and approval can I get a will from my grandparents. Is there any way for my aunt to break this will? 2. If there is no will on property and my aunt didn't claim her property share. After her life, whether my aunt's daughter can claim share?