Withdrawal of Power of Attorney as regards sale of land

There is an ancestral piece of land bought by my grandfather in the 1970s. After my grandfather's death in 1985, the ownership rights were handed over to his sons (3 including my father) & his wife (my grandmother). My father passed away in 2012 following which my mother replaced him in the ownership of that property. Upon finding a buyer for the property in 2016, two power of attorneys were created for competing the sale. One PoA holder was my father's elder brother and the second was a property broker (with no previous association to our family). My mother signed on these documents without going through the fine print owing to the trust she placed on my father's brothers and to top it, didn't even think about keeping a copy for herself. Come April 2017 and my mother discovered from a distant relative that the sale is now complete. She reached out to my uncles and my grandmother for an explanation and they refused to divulge any information. Upon pressing further, the younger son (my father was the second son) said that the sale was not complete and pretended not to know anything whether any payment had been received till date. My mother has now got hold of the PoA copies from the sub-registrar's office. What is it that we can do now, in the event that the sale is complete and in case, if it is still in progress? Please help.