"Makes it a folk song by at least one definition. W
So if someone gets up i a fol;k club and sings Puccini's Nessun Dorma it becomes a folk song - or when opera singer Kiri ti Kanawa sings Wouldn't it be Luverly, it becomes opera??
Silly definition - is it actually documented anywhere?
Bob Davenport used to sing Kurt Weill's,and Maxwell Anderson's September Song' at folk clubs ( basically to get up the noses of us "purists" I suspect) - a folk song - I don't think so.
Really don't know enough about My Brother Sylvest to comment - don't think there are hard and fast rules on individual songs - there are certainly borderline cases, but not, I suggest, among modern pop songs, for the reason I mentioned.
The point I am making about these songs is basically if you are going to understand them we need to be fairly specific on what we mean - they carry far too much historical and cultural baggage to just abandon an existing and well documented definition and not replace it with something we can all (in general terms) agree upon.
The present definition has never been seriously challenged - it certainly has never been replaced.
What has happen is that the a small number of people have decided that the terms 'folk' and 'tradition' have proved inconvenient to their particular interests and ambitions and have decided to hang their own particular flavour-of-the-month on a hook that has been occupied for well over a century and a half, without having the courtesy to explain why, other than "folk/tradition is what I choose to call it".   
It's far too important an art/cultural form to allow that to happen without a fight.
Sing what you want, but if you have any respect for the music, at least try to understand it, or allow those of us to wish to to do so without abusive terms like "purist" or "finger-in-ear" or "
"folk police" or even "folk fascist".
I think I know what folk/traditional song is - have spent over half a century trying to come to terms with it.
I out experience, the older source singers knew that the songs I believe to be folk/traditional were unique and had their own terms to describe what they sang.
If they were, I am wrong fine - show us where they/I am are and I'll make arrangements to pack of our large library of books on the folksong, ballads, folklore, music, dance tales, superstitions... (all related disciplines)   to Oxfam and fill the shelves with commended alternative - so far, nobody has come up with zilch.
Not very convincing.
Jim Carroll