If someone signed on behalf of someone else without an authorised power of attorney the Notary should have mentioned this in the escritura and that the brother has to officially confirm his agreement. The Land Registry should not allow a new escritura to be registered if they do not have a certified copy of the ratification or at least they will carry the same note in the Registry, i.e. that there is a missing confirmation from the brother. This could cause problems in the future. If the brother has since died then you would have to get his successors to confirm on his behalf.