Lyn, John Singleton Mosby's genealogy does not show any brother or any other relation named Edward. There are a couple of genealogical pamplets at the Queensland State Library, and in other sources, written about Edward Mosby, early in the 20th century, which do not even mention his supposed service in the Confederate Army. Additionally, there were about three or four persons listed with the name of Edward Mosby, who served in units from Arkansas and other states further south, but no Edward Mosby ever served in a Virginia unit, which was the state that John Singleton Mosby was from. All those from these other state units are known to have either died or remained in the United States, and none of them came to Australia.
If you do indeed have a photo of Edward in Confederate Army uniform, may I see a copy of it, please, as it will certainly prove that his claim was correct. Has it been confirmed as being a Confederate Army uniform, and not that of a sailor, or something else? However, all other evidence that I have seen, so far, tends to dispute his claim, or the claim of his descendants, if that was where it came from. So far, other than oral information, there has been no documented confirmation of his service in the Confederate Army. There were many who claimed to have served, but, unless there is documentation, some of these claims are rather spurious, and have been proven to be false.