Here's the entire text of the song (as I remember it):

I'm my Own Grandpa
Yes I'm my own grandpa
It sounds funny I know
But it really is so
Oh, I'm my own grandpa.

Many, many years ago when I was 23
I was married to a widow who was pretty as could be
This widow had a grownup daughter who had hair of red
My father fell in love with her and soon they two were wed

This made my dad my son-in-law and changed my very life
For my daughter was 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

My little baby then became a brother-in-law to Dad
And soon 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 grownup daughter (who of course was my stepmother)

(chorus)

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
For even though she is my wife she's my grandmother too

(musical interlude)

If my wife is my grandmother then I'm her grandchild
And every time I think of it it nearly drives me wild
For now I have become the strangest thing you ever saw
As husband of my grandmother I am my own grandpa

(chorus)

Clever, huh?

Mar"No mother-in-laws! Thank god!"kus K.