Joe: Yes, I believe the song is accurate IF you allow that a stepson can be called a son, a stepfather can be called father, and so on. If you insist on strict accuracy, that would require you to insert a lot of "steps."

Just for fun, I decided to do that:

This made my dad my stepson-in-law and changed my very life.
My stepdaughter was my stepmother, 'cause she was my father's wife.
To complicate the matter even though it brought me joy,
I soon became the father of a bouncing baby boy.
My little baby then became a stepbrother-in-law to Dad,
And so became my stepuncle though it was very sad.
For if he was my stepuncle then that also made him stepbrother
Of the widow's grown-up daughter who of course was my stepmother.

Father's wife then had a son who kept him on the run,
And he became my stepgrandchild for he was my stepdaughter's son.
My wife is now my stepmother's stepmother and it makes me blue,
Because although she is my wife she's my stepgrandmother, too.

Now if my wife is my stepgrandmother then I'm her stepgrandchild,
And every time I think of it it nearly drives me wild,
For now I have become the strangest case I ever saw.
As husband of my stepgrandmother, I am my own stepstepgrandpa.

Oh, I'm my own stepstepgrandpa
I'm my own stepstepgrandpa
It sounds funny I know
But it really is so
Oh, I'm my own stepstepgrandpa.

* * *
OK, I never heard of a stepuncle, or a stepstep-anything, but I believe my construction is logical. It might even get a few laughs to perform it that way, but only if the audience is already familiar with the song.

I know what you mean by being a literalist (although I think I grew out of my literalist phase by the time I reached 16). I do remember being bothered by the song TENNESSEE WALTZ when I was young. Assuming the song told a story that was literally true, how could she sing, "I was dancin' with my darlin' to the Tennessee Waltz" when this song WAS the Tennessee waltz? In other words, if it's true that someone really danced and then later wrote a song about it, and the song was called TENNESSEE WALTZ, then what song was she dancing to? OK, if you allow that the story was fictional, then it was a badly written one, because it was illogical and impossible.

Of course you can eliminate the paradox if you assume she only danced to the tune of the Tennessee Waltz, and the lyrics were written later, but I probably didn't know then that new lyrics were sometimes written to pre-existing tunes.