How to revoke or cancel poa

My family executed partition deed when I was only 10 years old. My dad is mentally unstable so my grandma was held as guardian(had right to enjoy the property- my grandma aandu anubavithu vanthu annar kaalam aana piragu my aunt paramarithu vanthu - should handover to me when I attain 18) for my share till I attain 18 years of age. My uncle got power of attorney from my grandma and sold my share to his son. She doesn’t know to read or write and didn’t know what she was signing. How can I nullify the POA? What actions can I take? Please help. I have no one to help and I need that property for my livelihood