Land dispute

My father owns a landed property by inheritance but another family entirely sold the land and the buyer claimed that my father's elder brother also sold the same land to him, And then the other family and my father's elder brother joined to issue him a receipt he alleged And all this is happening without my father's consent, which made my father sue the buyer and seller to court, that his elder brother did not sell any land. And this other family has no biological link to my family. Pls what is the solution