Property registration issue

Hello Sir My Grand Father bought a land from a person. He is the third owner of that property. we have registered this property on my mothers name and we have started the construction recently. Now, the problem is that, the wife of the first owner has filed a case against us saying, that property belongs to them. Even though we have shown the proofs, she is not ready to listen atleast. Dont know what to do next. Please suggest. Thank you Asha