Unmarried Maasi registered will probation

My mother's unmarried sister expired April 2018. She had no children, and her mother, father have also expired. She had two brothers and two sister, also all pre-deceased. One brother had 2 children, 1 was pre-deceased with no children. The second brother had 3 children, 1 was pre-deceased, but with 2 children. One sister was unmarried with no children and the second sister had 7 children. I am one of these children. Only one wife of one brother is alive. A long ago letter declared that my massi would leave her property to the one brother whose wife is still alive. But, 10 years ago, she made a registered will, leaving her property to me, her older sister's son. This was signed by 2 witnesses, still alive, but maybe not signed on the correct page. The society where my massi had her flat would not transfer the property to my name without a will probation because my one living maami thinks all the property should go to her and wrote a letter to the society saying as such. Since then I have tried the will probation, but my maami has filed a caveat. Can she claim any rights to my massi's property? What are the chances of overcoming the caveat?