Yes, very interesting anecdote.   As far as I can see the word 'flapper' was originally British slang and from another origin, but it's possible that when the word spread to America folk etymology quickly came up with an explanation for it connected with the fashion for wearing galoshes unbuckled.  This association must have happened early on, as the 1928 film 'Bare Knees' mentioned above seems already to reflect it. Could you add some dates to your anecdote, and tell us roughly when your grandmother was wearing this fashion?  BeyondMyKen is right, this is original research and we shouldn't add it to the article, but it could help explain where the 'galoshes' theory for the origin of 'flapper' came from.