Two Sisters tune and its "rain and the Wind" refrain were nicked (sorry - adapted) by Bob Dylan for "Percy's Song".

"All my Friends are Gone" (by Rev Gary Davis) is based on a true story of Delia Green, murdered in Savannah in 1899. This may not be old enough, but if you Google "Delia Green" + Savannah you will get contemporary newspaper accounts of the trial of Cooney Huston, which you can compare with the lyrics of the song. They differ considerably and in rather interesting ways. For instance, the song say that Delia was a gambling girl, but she wasn't. Older murders are unlikely to offer that facility. If the song changed the facts so much in such a comparatively short time, what relation might older songs have to any original facts?

Sorry this doesn't really answer your question but might be useful.

dj