Don't accept what the agent says without checking. I think the fact that your copy of the agreement was not signed for the landlord does not make it invalid, but you must get CAB or solicitor advice.
Also, I guess the original tenancy will probably have been replaced by a new one or become a "statutory periodic tenancy". Sometimes the wording of the guarantee means it only applies to the original tenancy. Again, you need this checked.
When you see CAB etc. make sure you take the guarantee document with you & - if you can get them from your brother - a copy of all the tenancy contract documents.