"Strange Family Tree" joke

Many, many years ago
when I was twenty three,
I got married to a widow
who was pretty as could be,
This widow had a daughter
Who had hair of red.
My father fell in love with her,
And soon the two were wed.
This made my dad my son-in-law
And changed my very life.
My daughter was my mother,
For she was my father's wife.
To complicate the matters worse,
Although it brought me joy,
I soon became the father
Of a bouncing baby boy.
My little baby then became
A brother-in-law to dad.
And so became my uncle,
Though it made me very sad.
For if he was my uncle,
Then that also made him brother
To the widow's grown-up daughter
Who, of course was my stepmother.
Father's wife then had a son,
Who kept them on the run.
And he became my grandson,
For he was my daughter's son.
My wife is now my mother's mom.
And it surely makes me blue.
Because, although she is my wife,
She is my grandma too.
If my wife is my grandmother,
Then I am her grandchild.
And every time I think of it,
It simply drives me wild.
For now I have become
The strangest case you ever saw.
As the husband of my grandmother,
I am my own grandpa!!!