Sold Property Issues

My mother had a residential plot which was sold 5 years back. It had some litigation then. But, it was successfully sold to some influential party which can manage it with power and politics. They picked it up knowing the challenges involved. Now we are out of touch with all concerned parties. Recently, we received a legal notice to the effect that we are creating some nuisance and it should be stopped immediately. Looks like the new buyer is trying to build and it is assumed that the land is still in our ownership. We have been threatened with criminal charges and also to bear legal charges of concerned party. Considering that we have nothing to do with the aforesaid property, should we update the law firm about change in ownership or should we just ignore it.