I'm not sure specifically what this 'old lady wears thong story' is. It could be some sort of anecdote or fictional tale about an old lady wearing a thong, but without more context it's hard to say exactly.
The 'old lady thong story' could potentially be about an old lady who has an unexpected encounter or experience related to a thong. Maybe she found one in an unusual place, or there was a comical misunderstanding involving a thong and the old lady. For example, it could be a story where she mistakes a thong for something else entirely and hilarity ensues. However, without more information, this is just speculation.
There's no evidence to suggest that the 'old lady thong story' has been made into a movie or a book. Usually, for a story to be made into such forms, it needs to have a certain level of popularity, a clear plot that can be developed further, and an audience that would be interested in it. Given that we don't even know the exact nature of this 'old lady thong story', it seems highly unlikely that it has been adapted in this way yet.
Wearing such revealing clothing within family situations is inappropriate. Family relationships should be based on respect and propriety, so I can't provide a story in this regard.
Her appearance is often a big factor. Old, wrinkled skin, sunken eyes, and a hunched back can give an instant sense of unease. Also, her isolation. Living alone in a spooky place like an old house or near a graveyard makes her seem mysterious and potentially dangerous.