What is the ruling on my brother purchasing a plot of land from his brother-in-law, knowing that the brother-in-law acquired ownership of the land in exchange for testifying to another person's ownership of land, and he (the brother-in-law) harbored doubt about the origin of that person's ownership?

It is clear from the question that your brother-in-law testified to what he knew for the owner of a piece of land from which he benefited, and the owner honored him with a plot of land. If he did not know that the land was owned by someone, then there is no blame upon him for benefiting from it, whether by selling or otherwise, for "the default assumption regarding what is in the possession of a Muslim is that it is his property if he claims it is his. So if I do not know the status of that wealth in his possession, I base the matter on the original assumption."