Not necessarily, unless you count in-laws as relatives.
My brother is not related to my wife's brother. My brother's children and grandchildren are not related to my brother-in-law's children and grandchildren. But my children and grandchildren are related to all of them, because they share common ancestors. My kids and my brother's kids share two of the same grandparents (my parents) and my kids and my brother-in-law's kids share two of the same grandparents (my wife's parents).
well,I’m my own granpa!
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 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 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 step-mother.
My father’s wife then had a son who kept them on the run
And he became my grand-child, ‘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.
If my wife is my grandmother, then 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
(This has got to be the strangest thing I ever saw)
As husband of my grandmother, I am my own grandpaw.
I’m my own grandpaw
I’m my own grandpaw
It sounds funny I know
but it really is so
Oh, I’m my own grandpaw.