Distress in seling land

My mother has a plot which was picked up 22 yrs back and tax has been paid from then onwards after the site was registered in her name. It was purchased from a gentleman who has mentioned in the agreement/stamp paper document that he inherited the land from his father and that the land belongs to him. He has signed as the owner of the land in all documents. We now plan to dispose the plot being held by my mother. Upon enquiring with brokers we were told that all plots in that particular area have a problem, as the gentleman’s (from whom the land was purchased) brothers are now saying that they too have a share in that land and are sending lawyer notice to this effect whenever someone is trying to sell their plot. One or two parties have apparently settled the issue by paying a lumpsum amount to the brothers and their children. This amount runs into lakhs of rupees. The real estate agents we contacted mentioned that all 3 brothers signature is required to sell this land. Not sure if this is a money making gimmick or whether there is any legal angle at all. The brokers have asked us to show them the property papers so that they can take a look and let us know. Ideally, we would not like to share the property documents with a third party, as we are not aware of their intentions. We have checked the papers and found that it only contains the signature of the person from whom the land was purchased. Request you to pls advice us. Thanks in advance.