Are we allowed to testify that my brother bought the gold jewelry and that his wife was wearing it when she left the marital home, knowing that its real value is not 40,000 pounds as she claims and demands, and my brother signed the list without knowing its contents?

It is not permissible for you or your wife to bear false witness. The justifications you mentioned do not make it permissible to proceed with it, and dissimulation is not beneficial in such situations. The injustice to your brother is not certain, so we advise both of you to refrain from testifying to what you do not know.