I've written this from memory - I think it's right!

Many, many years ago, when I was twenty-three,
I was married to a widow, who was pretty as can be.
This widow had a grown-up daughter who had hair of red.
My father fell in love with her and soon they too were wed.
This made my dad my son-in-law and changed my very life.
My daughter became my mother, '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.
This little baby then became a brother-in-law to Dad,
And so became my uncle – oh, it made me very sad -
For if he was my uncle, then that also made him brother
Of the widow's grown-up daughter, who of course was my stepmother.

I'm my own Grandpa. I'm my own Grandpa.
It sounds funny I know, but it really is so.
I'm my own Grandpa.

Father's wife then had a son, who kept them on the run,
And he became my grandchild, 'cause he was my daughter's son.
My wife is now my mother's mother, and it makes me blue,
Because although she is my wife, she's my grandmother too.
Now, if my wife is my grandmother, I am her grandchild,
And every time I think of it, it nearly drives me wild,
For now I have become the strangest case you ever saw:
As husband of my grandmama, I am my own Grandpa.

I'm my own Grandpa. I'm my own Grandpa.
It sounds funny I know, but it really is so.
I'm my own Grandpa.