Family property dispute

Father filed a suit on his son to want his property back which he had earlier transferred to his son claiming that the son had forged him and he never wants to give him the entire property.During the suit father got expired and daughter moves an LR application in to the court and makes both son and daughter a plaintiff.Now son who was originally defendant in the suit becomes both plaintiff and defendant.Is this application in the court is maintainable.If not ,what are the remedies available to the son as an defendant AND daughter as an plaintiff.Please guide