The Hapsburgs: it's all explained here

Many, many years ago when I was twenty-three

I was married to a widow who was pretty as could 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 really changed my life

For now my daughter was my mother, 'cause she was my father's wife

And 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 so became my uncle, though it made me very sad

For if he were my uncle, then that also made him brother

Of the widow's grownup daughter, who was of course my step-mother

Father's wife then had a son who kept them on the run

And he became my grandchild, for 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, then I'm her grandchild

And every time I think of it, it nearly drives me wild

'Cause now I have become the strangest 'case you ever saw

As husband of my grandmother, 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