"The 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 gorgeous 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,
For daughter was my mother
Since she was my father's wife.
To complicate the matters more
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 this 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 step-mother.
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 really 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!