What can we do if our father's brother, the executor of my father's Will, throws away items of value that were our father's without our knowledge?

The executor does not have authority to throw away items.  If he will not stop, you will have to petition the probate court to order him to stop.  An executor who does this can be removed by the court or denied the right to serve as executor.  I suggest you consult a probate lawyer in your father's state to assist you.