What is the validity of sale agreement with possession?

My paternal grandfather had 300 sq. yards of land in Hyd, and that was transferred to my mom's name (daughter-in-law). My uncle (mom's brother) has cheated us and sold out our land. Without knowing her, he has taken her registration office at closing time and signed the registration paper, and we don't even know at what price he has sold. Everything he said lied as per his pre-planned chakravyuh. Now I just want to know if "Agreement of sale cum general POA with possession is valid or not in India". Person who purchased from us (vendee) has done AGPA with possession instead of sale deed. Vendee sold out the land after 6 months to another person, but vendee (purchased from us) didn't take our permission or sign of my mother, as she is principle holder. I've read some cases in which clearly written principle needs to be signed while transferring property by vendee. Please provide , Is AGPA with possession is valid? Is principal holder's sign required while transferring property by vendee to others?